LIGHT AND CAMERA INTERACTION CONCEPTS

LIGHT AND CAMERA INTERACTION CONCEPTS

BASIC CONCEPT: WITHOUT LIGHT, THERE IS NO IMAGE.

Virtually anything that the human eye can see in the visible spectrum(400 to 760 nanometers (nm))can be recorded as a video or still image.

The proper choice of video cameras and lenses can extend that range down into the ultraviolet (below 400 nm) and up into the near infrared (above 760 nm). For special applications, special imagers with sensitivity in the range of 7 to 15 microns(7000 to 15000 nm) are available to image emitted (sentient) body heat of warm blooded animals when there is sufficient thermal contrast with the background.

CCD (Charge Coupled Device) video cameras are inherently more sensitive to infrared light than visible light. Therefore color cameras must filter out (or cut) infrared light to properly balance the visible part of the spectrum to create a color image. Color cameras are available that can image the full visible color spectrum under white light conditions and when required, physically remove the infrared cut filter from the light path allowing sensitivity into the infrared as well as any remaining visible light. This function will usually increase the camera sensitivity by a factor of 10.

Monochrome cameras are inherently more sensitive and have higher image resolution than color cameras because each pixel is sensitive to the full spectrum of light. Color cameras require selective filtration (red, green, blue) over each pixel to proper render a color image.

The human eye is wonderfully adaptive when viewing virtually anything. It can adapt to illumination levels of over 10,000 to 1, easily be moved to optimal positions, ignore glare and compensate for various backgrounds.

Cameras are basically pretty dumb. Simple cameras interpret the world as an all gray background with 18% reflectivity (average reflectance of a Caucasian face). Therefore if you point it at a white surface, the camera reduces the amount of light falling on the imaging chip by increasing the electronic shutter speed (or reducing iris size) and outputs the image as 18% gray. If you point it at a black surface, the camera increases the amount of light falling on the imaging chip and outputs the image as 18% gray. Now you know why simple cameras cannot properly image small areas of interest when 80 to 90% of the imaged background is either very bright or dark. You have to move in close. A simple camera compensates over a light to dark image area ratio of 8 to 1. Above 8, everything is white, below 1, everything is black, 7 to 2 are shades of gray.

Technology to the rescue (partially). A new generation of cameras with Digital Signal Processing (DSP) can be manually programmed to interpret the light being reflected off of only a portion of the image area while ignoring the glare or shadows in other area of the image. It is generally referred to as Back Light Compensation (BLC) from it origins in the security industry. The very latest generation of cameras can automatically perform DSP on every pixel in the image area thus increasing the dynamic light compensating ratio to 128 to 1. However, it still cannot compensate for fast moving reflections like ripples on a water surface because of damping factors built into the DSP system.

LIGHT

"WHITE" LIGHT

White light is the sum of all portions of the visible spectrum which are available to illuminate the video subject and that portion of the subject which reflects all of it is seen as "white". Subjects which reflect a portion of the spectrum (say red) are interpreted as being of that color (red) and they absorb the other portion of the visible spectrum. Some subjects which appear in the visible spectrum to be dark (especially dark green leaves) when illuminated with infrared light appear almost white because the chlorophyll in the leaf reflects virtually all of the infrared light. Critical observations based on subject patterns or markings, which require infrared illumination, require testing to see if those markings can be differentiated under that type of infrared illumination.

All "white light" is not the same especially when viewed by a color camera. Portions of the spectrum may be missing. For example White Light Emitting Diodes (LEDs) are actually a combination of a blue LED emitter under a phosphor coating that when "excited" by the blue light and filtered by the coating create a "white light" as interpreted by the human eye.

Likewise fluorescent lights appear to be white but the phosphors can be biased to provide warm light, cool light, "natural" light or daylight spectrums but they are not the continuous spectrum which incandescent lights provide. An additional problem with line powered (60 HZ) fluorescent lighting is that it is slightly out of synchronization with a camera's base frequency. This causes a color image to slowly alternate between a cool image (bluish) and a warm image (reddish). Certain cameras can cancel out this fluctuation while other cameras require special high frequency (20 KHZ) fluorescent lights.

Incandescent lights generally provide a continuous spectrum of white light. However, various types are available to provide warmer or cooler illumination. Deviations from the design voltage of the incandescent light can also bias the spectrum emitted. If accurate color rendition is required, high quality quartz-halogen incandescent lights are the illumination of choice.

Outdoor "white" daylight is also not constant. Warm early morning light may have a spectrum centered around 2400º Kelvin while noon daylight with a blue sky may be 12000+º Kelvin. Your eyes compensate automatically but a camera requires "white balance compensation" if colors are to be recorded accurately.

Infrared illuminators are, in general, the spectrum of choice when humans and other vertebrates are to be observed covertly and not bias the viewing of those subjects. In addition, infrared illumination will not attract unwanted attention by non-targeted subjects and predators. However, IR illumination can easily be observed with various "night vision" devices.

For most applications, infrared Light Emitting Diodes (LEDs) provide sufficient illumination to distances of 150 ft. (50 m). For extreme distances, long pass filtered (far red and infrared pass through, visible is absorbed) incandescent line powered (120 VAC) illuminators are available.

Two groups of infrared LED's (840 to 880 nm and 940 to 950 nm) are available and the choice depends on the level of covertness required for the illumination system. 840 to 880 nanometer (nm) are generally chosen because the cameras are more sensitive to that portion of the spectrum. Even though 840 to 880 nm is beyond the 760 nm vertebrate perception level, the emitted spectrum is not all in the 840 to 880 range. The emitted spectrum is a bell shaped curve centered at 840 to 880 nm but it tails down to below 760 nm. If the LED emitter is fairly close to the vertebrate observer, many tiny reddish "eyes" can be seen against a dark background and may bias the observation.

If these tiny "eyes" are a potential problem, the solution is use LEDs with peak emission in the 940 to 950 nm range. With these emitters there is no "spillover" into the visible spectrum and they are totally covert for all know vertebrates. If the emitter is very close to the subject, say 1 ft. (30 cm) or less depending on the number of emitters, some low level heat (far infrared being radiated by the LED housing) can be felt but no visible light can be seen.

For underwater observations in the infrared, the 840 to 880 nm are the emitters of choice because their spectrum is not as rapidly absorbed by the water and they are generally usable to 3.3 ft. (1 m). The 940 to 950 nm LEDs can also be used, if required, but the working distance is even shorter.

When observing invertebrates, red LED illuminators (640 to 650 nm) can generally be used with either monochrome or color cameras. Working distances to 10 ft. (3 m) can be achieved underwater. Caution is advised, when using red illuminators during underwater observations; Examples: 1) Those involving both vertebrate predators and invertebrate prey; 2) Light spillover may make target species reluctant to move through the imaging area.

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