WP's SloMo

User know-how

Color CMOS sensor

Environment illumination provided as standard in an office or in a production hall is almost sufficient for most black-and-white video high-speed camera systems. They have sensitivities somewhere between about several hundred and several thousand ASA at nominal frame rate depending on amplification. Color versions usually show about 25% of that only.
The special features in comparison to standard imaging mainly derive from the usually short exposures and the working close to limits - physical and technical ones.
Otherwise obviously the usual photographic rules, restrictions and, of course, possibilities are valid.

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Tips for shooting

Illumination, aperture (f-stop) and dynamic

Evidently it is valid: everything counts in large amounts. The more light is offered the more the aperture can be closed, i.e. a higher f-stop number can be set, enhancing the depth of field. At f-8 or higher it is well done, but 5.6 is sufficient enough.
Usually there is never too much light - it is possible to shoot the glowing filament of a 500 W tungsten bulb, the hot spot during laser welding or an electric discharge. (Caution, directly viewing in the laser beam is then too much of a good thing.) Only the brightness dynamic of the camera is problematic. That is just valid for all electronic cameras. Good to notice when making a photo looking outside a window deep from the room. (Therefore in interviews in office rooms one prefers to shut the curtains or jalousie. Besides then the spotlights are not so intensively reflected. Until HDRI (high dynamic range image) is widely spread some time will pass.) The usual 8 to 12 bit or 256 to 2 048 brightness levels, should be optimal used, even if 64 gray tones (6 bit) provide a sufficient quality. If one wants to image the less illuminated environment, one will have to light it up. Comparable to the additional (background) light-up flash in photography.
Just to mention, if it is too bright at all, (neutral density, short: ND; i.e. neutral, colorless) gray filters will offer help. Or selective filters, which e.g. dim the wavelength of a welding laser, but not those of the weld deposit. Many lenses provide threads for that reason.

Practice-oriented example: if one wants to image the sparkles of a cigarette lighter together with the holding hand, about 1 000 W will be necessary (see sample 2 with and 3 without lighting up in [SloMo Clips]).

By the way: the dynamic range is often given in units of decibel [dB]. The connection to bits and bytes gets clearer with the following formulas:

x [dB] = 20 × log y and y = 10^x/20, resp.

In which x represents the value in dB and y the number of sensitivity steps.

Thus 8 bit (= 2⁸ = 256) make so 20 × log 256 = 48 dB, while 60 dB stand for 10^60/20 = 1 000, so for about 10 bit (2¹⁰ = 1 024) dynamic range. Thus 12 bit would be about 72 dB. These are typical values.

As technical expression here the term signal to noise ratio is to mention. It indicates how much stronger the useful signal »exposed image« is compared to parasitic effects - not in the latest charges of a pixel (in real a photocell; ~»solar cell«) are even spontaneously and arbitrarily generated without any illumination but caused thermally only. And the term full well capacity indicating how much charge a photocell can generate at all until saturation takes place, thus more incident light does not generate further charge carriers. Here large area pixel show their advantage concerning dynamic.

Click for flicker illumination effect: [SloMo Freq.]

More about light -
Photo hf:

Photo

Professionals use at least halogen spots in studio quality (defined color temperature ...), cool beam reflectors (with reduced IR spectrum), or even HMI spots (daylight spectrum) and DC illumination devices (rectified) to avoid the jitter of the 50 Hertz or 60 Hertz, resp. (1 Hz (Hertz) = 1/sec), power supply frequency.
There are obviously lamps switching on and off during operation and one usually does not take notice of it. Even light emitting diodes (LED) can flicker, partly unintended due to a not sufficient enough stabilized DC power supply. But partly also intended - the maximum current, and thus the (momentary) brightness of the LEDs in flash mode, is namely allowed to exceed that of continuous operation by far. (Suitably synchronized they are appreciated because of their lack of radiate off heat.)
A sequence mainly illuminated by common fluorescent tubes therefore may show distinct pumping brightness, see the figure on the left and for details see [SloMo Freq.]. The effect is also known from flickering slow motion replays on TV broadcasts. (Obviously there were some reasons for the good old light bulb. ;-)

Due to the different color temperatures of light sources, e.g. the tungsten filaments of halogen lights shine with longer wave-length or warmer (i.e. more yellow,reddish) than daylight, electronic cameras offer a so-called white balance feature. Then white will become pure white again.

And caution: a halogen spotlight with a nominal power of 1 000 W heats with about 900 W. This might be sufficient enough to melt plastics within a distance less than 1 m after a while!

Exposure and aperture (f-stop)

Exposure, better the maximum time of exposure, of most high-speed cameras, is nearly 1/frame rate. With e.g. 1 000 frames per second, thus the time of exposure cannot exceed 1/1000 second. The sensor is integrating (= gathering) over the incoming illumination during this span of time.

While the f-stop in general limits the light intensity for exposure the shutter is mainly used in order to reduce distortion caused by motion (motion blur). Because if the object is moving during the time of exposure with more than 10% of its size, this will be bothering usually. (In still photography one accepts just up to 3%, if ever.)

There are different possibilities for reducing motion blur:

• Increasing the frame rate (may cause a decrease of resolution due to camera possibilities and a demand for more light)

• Use of a stroboscope (where the background illumination must not be too bright and the area illuminated by the stroboscope is limited)

• Use of an optional shutter, e.g. a liquid crystal shutter (LC shutter), which permits exposure down to about 1/10 000 sec, but causes the loss of two aperture steps even totally opened (transmission typically about 30%).

Many up-to-date systems offer as standard a more than ten times faster electronic shutter in their chip design and therefore do not need an additional LC shutter. Exposure times of microseconds and less are not technical problems, but export topics addressed by dual use restrictions (military use and proliferation).
For those, who want or need more - in this case shorter exposure times - a Kerr cell or a Pockels cell, or possibly a microchannel plate (MCP) can help. Of course, there are mechanical shutters and choppers in the shape of slit or hole wheels, too.

Annotation: in contrast to the mentioned above global shutter (or snap-shot shutter, freeze-frame shutter) simultaneously operating on the whole sensor, there is also a line by line operating rolling or slit shutter. Concerning movement analyzing one should prefer cameras with the first one, because the line scanning of the sensor does not guarantee to take the resulting frame at a defined moment and according artifacts can occur.

Changing the shutter time one can vary the illumination, of course, if one is not able or does not want to adjust the aperture (f-stop) ring. The latest concerns more the artistic approach, because the depth of field depends on the aperture. It increases with the f-stop number. Who ever may need it...

Depth of field (or depth of focus)

Expression for the distance region where objects appear sharp on the image. The reason why it is not just a simple point like the inscription on the distance ring (focus) seems to claim, is that within a tolerance area the film, the sensor or one's eyes do not show resolution enough. When a point is displayed within the so-called circle of confusion (about 0.01 mm to 0.025 mm diameter with common sensor formats and 0.033 mm with 35 mm format cameras), one will not perceive the blur at all.

The position and expansion is mainly dependent on the aperture: small aperture (= higher f-stop) number - large depth of field. Where it is larger behind the calculated position of focal distance than in front of it.
Depth of field increases with shorter focal length and greater distance to object. Larger film/sensor format exact larger pixels, resp., however, permit to decrease it. (Due to crop factor sensor shrinking increases depth of field by force.)

There is a rather bulky formula, see distance and focal length calculator [SloMo f = ∞]. Anyway depth of field is a no-word in high-speed imaging in industrial environment (mostly the aperture of the »Dark-o-matic« is just opened like a barn door. ;-).

Using the so-called hyperfocal distance h the region starting at h/2 to infinity appears in focus. It is also called fix focus setting or near adjustment and is especially used with simple cameras.
Cheap lenses, especially for surveillance cameras, often do not even offer a focus ring. They are focused by the aperture only. Here the electronic amplification (gain) of the cameras has to provide the required brightness.

Field of view and lens

Endeavor to do a good job: image the interesting scene as screen filling as possible. With C-Mount it is not too tricky to use spacers (extension tubes in a set for less than €/$ 50.-) and a fixed focus lens or an additional short distance lens for covering the entire screen with an object of 5 mm (= 1/5 inch) in diameter. Do not choose a wide-angle lens with a focal length smaller than 6.5 mm (concerning C-Mount 2/3 inch format) for applications using automatic image processing (e.g. with object tracking), otherwise distortion would reach a level not to be tolerated. All lenses and equipment suitable for the according mount can be used, even filters, macro, zoom and fish-eye lenses and with appropriate adapters photo lenses, bellows, microscopes, endoscopes, boroscopes, fiber optics, image enhancers (night goggles) ... But pay attention to the fact the more complicate the optics are the more illumination is often necessary.

Professional photo stores offer adapters to fit common lens mounts, e.g. C-mount to Nikon bayonet (Nikon F). Some high-speed cameras (like many professional photo cameras, too) additionally allow - due to a mounting plate/reference plane - the choice of a wide range of (customer) specific adapters including popular ones like Nikon F or C-Mount and others e.g. PL, Kinoptik or Stalex (M42).

Click for focal length calculator [SloMo f = ∞]

Actually the focal length inscription on a lens is the focal length at the image side, if imaging an object in infinite distance with a wave-length of 546 nanometers. Then the image will appear in the focal point located at the image side of the lens.
The definition according to DIN 4521 standard is:

f' = lim_ω-->0 (y'/tan ω)

With the half field angle ω and the half image diagonal y'.
(Suitable for practice subsequent the field angle is simplified to be the angle of view due to format filling setup and f' and f are just taken as the focal length inscription of the lens.)

The rule of thumb for a rough calculation of the focal length for a format or a sensor filling image S is

focal length = distance to object / (1 + object size / image size); [all values in mm]

And the estimate of the necessary distance at given focal length is

distance to object = focal length × (1 + object size / image size); [all values in mm]

The angular field of view FOV = 2ω (≡ 2ω', see the figure above) is given by

FOV = 2 × arctan (1/2 × image size / focal length)

The magnification M derives from the imaging through the lens onto the sensor and the display of the image on the monitor or other media

M = focal length / (distance to object - focal length) × diagonal of monitor / diagonal of sensor; [all values in mm]

One must pay attention to the restrictions of standard lenses. In a distance less than 0.3 m (approx. 1 foot; sometimes even 1 m, approx. 3 feet) to the object they cannot provide sharp images. In such cases one needs a spacer (= extension tube), an additional short distance lens or a micro (-scope) lens, and so on.

The thickness t of the spacer, which is screwed in between lens and camera body, is given by

t = image size / object size × focal length; [all values in mm]

in which the relation image size / object size is called imaging scale. But caution, just despite of their simplicity - extension tubes swallow light.

Fine adjustment of flange-back (back focus lens adjustment)

Evidently a wrong adjustment of flange-back derives from manufacture tolerances, mismatch (incompatibilities of components) and media brought in the beam area between lens and film or sensor, resp. And concerning crash-proofed cameras mechanical stability is above all, even above (too sensitive) adjustment mechanics.
Provided that the suitable adapter is selected using a lens of fixed focal length one easily receives sharp images by turning the distance ring (focus). Maybe the distance inscription and the magnification are slightly wrong then. Usually this does not bother one further more.
Using a zoom lens, however, one looses the sharpness (i.e. focus) of the image during zooming process. As a rule it should be steady and only magnification (and thus the angle of view) should change. Then the zoom lens is only usable in a restricted manner as vario lens. One is ought to adjust sharpness simultaneously during zooming all the time. High-speed cameras, however, are rarely used for such zoom shots.
Nevertheless, in order to take full advantage of the zoom feature the flange-back has to be adjusted accurately. Using simple tools proceed as follows:

1. Open aperture as wide as possible to reduce depth of field (if necessary dim the room illumination, reduce time of exposure, ...)

2. Select an object in a distance of about 3 m to 7 m (this is about 10 feet to 23 feet)

3. Gain a sharp image at maximum zoom (biggest focal length) by turning the distance ring (focus)

4. Gain a sharp image at minimal zoom (smallest focal length) by changing flange-back. (Do not turn the distance ring hereby)

5. Iterate until you receive sharp images at both zoom positions without re-adjusting

To adjust flange-back camera housings offer either a thread tube to be moved back and forth - almost standard with C-mount cameras. Or metal pads (washers) are fed in between lens adapter and camera body, if not even mechanics are integrated in the camera body in order to change the sensor position. There are also lenses, especially C-mount ones, with a cylinder housing at the camera side, which can be shifted. Look for a small depth bold at its circumference.
The IR switch of high-end lenses does nothing different. It quasi moves the lens away in order to project the IR images onto the sensor.

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Evaluation and measurement

Eminent: please do not forget you receive a lot of data. A megapixel resolution at 1 000 frames/sec and more just leads to data rates in the Gigabyte/sec range. Thus more than a complete CD-R per second would be filled. Per each camera, mind you. (Here at [SloMo Data] you will find the formula to estimate the data amount.) So do not wonder why the high-speed camera system is rather busy, when downloading, saving and viewing these files. And - it is highly recommended to have a storage/backup concept for the files.

The huge data amounts cause the cameras to be used offline. Thus they are not immediately used for controlling and they are not directly integrated in a superior machine control circuit. The image processing would be just too costly and too slow. One watches the scene and analyzes afterwards.
(Today slower cameras of the image processing sector - »machine vision« sensors - can already be equipped with a lot of calculating power, therefore named smart cameras, allowing them to work like a sensor only providing a simple good/bad signal for the control unit - e.g. »Label position on the bottle is all right - Yes/No?« - and not sending image data for further computing.)

Trajectory evaluation: translation, rotation,
velocity, acceleration and stick-figure animation

For controlling high-speed cameras even multi-channel systems of different suppliers special software is available. The evaluation is carried out either directly visual or using motion analyzing software packages, so-called motion trackers. See e.g. the links given in [SloMo Links].
To make it easier for automatic motion tracking by software one should pay attention to ensure a homogeneous background and, if possible, to avoid gratings, chessboard pattern or something similar (like a wallpaper with flowers ;-). Thus reducing the calculating time and preventing the tracking algorithm getting stuck with the attractors in the background instead of pursuing the desired target.

D-Load

If you intend to archive your image files in AVI format, consider making use of the compression tools Intel Indeo or DivX saving up to 90% memory capacity without loosing too much quality. DivX will often provide smaller files, especially if less movement happens in the scene. In contrast Indeo commends itself for automatic image processing, because it manipulates object position in a smaller extent.
AVI files can be processed (i.e. changing frame format, replay speed ...) with e.g. video editing soft-/freeware. For these and other tools just have a look here in the download center, see the button on the left.

Namely one shoots sequences with super slow motion to slow down fast movement for visual inspection, nevertheless, one should create some faster replays, e.g. with 25 to 100 frames/sec (of an original with 1 000 frames/sec), otherwise the impression of movement is lost. This will become important then, if one wants to show the sequences to some outsiders, who are not so familiar to the scenery.

Because the high-speed cameras run also with a normal speed of 50 or 60 frames/sec it is also possible to shoot true video clips. And copied on a USB stick with the same replay speed one can hand out it to the customer the machine has been built for with the words »So your machine has been working during the final tests«. Just generating a sophisticated impression, not only with the high resolution cameras. Thus the test becomes a advertising movie.

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Tricks for system integration

Trigger and synchronization

Easy to understand - to trigger means nothing else, but to start something caused by a single event. For instance the recording will be started, when the crash test vehicle hits the wall. The trigger device is often just a simple closing contact at the bumper providing a short circuit in the impact moment or a light barrier. (Often a flash light additionally marks »frame 0«.)
To synchronize means to stabilize the frame rate in a defined relation and a fixed delay towards a repeatedly happening event during a period of time. Ideally this is done by a recurring control signal, which causes a frame to be captured each time. So using a stroboscope as illumination source one will adjust the record phase of a camera so that it will be in recording mode, when the stroboscope flashes. So one equals the frame rate and the flash rate and adjusts the image capture to ensure the camera is active during the flash, not that its shutter is just closed, thus it is able to catch the rather short flash.
Sometimes cameras offer a so-called »strobe« control signal. It marks the phase of exposure of each frame; during the cameras is in exposure mode, it is set.

Adjustable delay trigger

Triggering is not as simple as it seems at all. Because the synchronously operating camera has to react on a sudden asynchronous trigger impulse after all. Therefore a capture gap can easily occur, because the camera has to finish the previous image capture before. Due to possibly already writing in the image ring buffer memory when the trigger impulse comes high-speed cameras often do not show a so-called restart capability like video cameras without image memory can offer. The latter are able to start a new frame almost in time with the trigger. Earlier image data are discarded where necessary.)

If one wants to trigger or synchronize various devices, one will face the problem of different signal levels, impulse widths and phase shifts not willing to work together.
Therefore here on the left side a very simple and cheap circuit, see [SloMo Trig.] for explanation, which may be helpful for some adjusting jobs.

(Because not everywhere »Trigger« is labeled, trigger is really inside. ;-)

Just in multi channel and especially in 3-D measurement applications synchronization between the cameras and their triggering in general becomes extraordinary important.

In frame trigger and external trigger

Engineering and CMOS make it possible. The trigger is released by the frame content as soon as a predefined color change of a certain pixel count surpasses the threshold value in an also predefined region of the (live) image. Elegant, but what to do if it is necessary to react on an electrical trigger signal from outside, but there is no interface available for it - e.g. at the smartphone? Just place a LED into the scene and operate it by the external trigger. That is then the useful trigger region. A small delay may occur. Well, possibly a nice job to measure it?!

Remote control

Exercise: a camera head should be adjusted (scene, aperture, sharpness, shutter, trigger, format ...), but the host PC is in a distance, e.g. in a control room behind a safety door, so that no live image is available near the camera. Or one wants to control the camera system from an extended distance.

The common interfaces (Gigabit Ethernet, etc.) are sufficient fast enough, but one is often dependent on expensive control software of the camera manufacturer or a third party provider. (And WLAN is tricky thing - either the camera is not equipped with it or one is a little bit scared due to the sensible data.)
Why you do not just try an inexpensive KVM? A keyboard-video-mouse extender (or switch) offers an efficient remote control without interfering with the PC. Even additional driver/software are not necessary.
The KVM consists of a transmitter and a receiver in the simplest case connected with a standard Cat 5 UTP Ethernet cable. The transmitter is plugged to the control host PC instead of keyboard, mouse and PC monitor. The real peripheral devices are connected to the receiver. Now one can operate at the receiver as if the host PC stands beside one's knees. Depending on the chosen system or transmitting technology, resp., distances of several ten meters up to some hundred meters and even more are possible.
Meanwhile All Share®, Airplay® or whatever may be a substitute. Even a WLAN monitor. But do not underestimate the delay for sending and displaying a frame through air, especially if you want to use it for triggering by hand. Test it!

Autologon and automatic start

Exercise: a Windows PC with or without Ethernet connection should start alone. In a special case even without connected keyboard, mouse and monitor.

One can bypass the password query using the autologon feature of Windows. (Attention: after that the password for the system and the network stands in the regestry.) One creates a new user account without password query or one sets an exiting account accordingly.
If desired, put the program (or its shortcut) you want to start automatically in the Autostart folder to be found in the Start menu programs.
Additionally activating in BIOS »halt on no errors« causes the PC not to wait on a feed-back of a keyboard or a mouse. This will make sense if you can operate the system by a remote control.

Result: the system runs up to the Windows Desktop, and with your desired program in the Autostart folder, even up to your application.
Visit the Windows Help if necessary.

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Cleaning lenses, sensors and LC shutters

Experts at work - of course, nobody has grasped on the lens and even less one has left the lens uncovered until a regular dust hill has gathered on the top. Blow off would not help yet and one has too often the impression to distribute the dirt only during a cleaning attempt at all. Not to mention the risk of scratching the comparatively sensitive anti-reflex coating, when using a dry duster.
Alcohol (isopropyl) drenched cotton pads, damp cleaning rags for spectacles or chamois leather with water thinned dishwasher or window cleaning detergents (and soft wipe paper to remove their residues!) are much better. The ultimate tool against fingerprints and dirt on glass, however, is e.g. First Contact Polymer (substitutes Opticlean Polymer from Dantronix) - not quite cheap, but final; or just even use the lens hoods instead ;-).

Usually the sensors of video cameras are protected by a sometimes coated, i.e. with optical layers, glass plate. Especially in still cameras and black and white configurations, however, the chip can be plane open. Then caution should be exercised when cleaning due to the sensitive color pattern film (polymer) or the sensor surface and its bond wires. It is highly recommended cleaning the LC shutter, if ever, with much more sensitivity than one is used to do so with glass surfaces.

When no voltage is supplied to the LC shutter, it may show spots and blots making it look spoiled. Do not worry, the operating voltage will erase them all. You can easily check this by operating it at the camera with low frequency and trying to look through it. Or by supplying alternating DC voltage (±5 V? see manual!) to the shutter and looking through it.) Then it completely opens and completely closes.