IR ILLUMINATOR

An infrared Illuminator can be easily made from a few infrared LEDs and resistors that will enable illumination of small to medium size areas up to several hundred square feet. Most Digital and CCD video security cameras are sensitive to near infrared wavelengths in the 700 to 1100 nM range just below the visible light spectrum. The visible range runs from about 400 to 700 nM, with violet at the 400 nM end , running through blue, green, yellow, and red at the 700 nM end.

Ordinary digital cameras used in photography are also IR sensitive down to about 1100 nM, since they use CCDs or CMOS sensors. Modern digital cameras incorporate IR blocking filters to get rid of ambient IR light which would otherwise cause distorted color rendition in photographs. It is quite possible to replace the IR blocking filter with a filter designed to block visible light and pass Infrared light only. Such a digital camera can be used to take infrared photographs without the use of ridiculously expensive, fussy to process, grainy, hard to handle infrared film, with far superior results in quality and sharpness. Applications include art and scientific photography, astronomical work, and forensics. There is a company called Lifepixel that offers such a conversion service for digital cameras, with excellent results, and should you need an infrared digital camera, this is the place to go. Two Infrared photographs taken with a converted digital camera (Nikon D100) are shown in the examples. The cost of conversion will be quickly recouped, as IR film is going for $10 - $12 or more per roll and must be home processed. These illuminators to be described might prove useful for surveillance photography with infrared digital cameras that have been converted to IR.

Typical LEDs produce 470 nM (blue) 525 nM (green) and 625 nM (red LEDs). Infrared LEDs produce longer wavelengths, with 880 nM and 940 nM being commonly available. Most CCD cameras are a little more sensitive to 880 nM although when these LEDs are used for security applications, some individuals can detect a very dim red glow from these. The 940 nM LED radiation are completely invisible to the eye and work almost as well. Some of these LEDs are clear, while others are tinted with pale shades of orange, red, dark gray or blue, or even black. They come in various configurations and radiation patterns, but 5 mm types with 15 to 40 degree patterns are probably the most common types. Power levels are a few milliwatts, and are specified in milliwatts per steradian (a solid angle). Narrower beam units will appear to have a higher power output as the energy is more concentrated in the narrower beam width, much like a sharply focused flashlight. Typical Infrared LEDS operate at 20 mA and 1.5 volts forward voltage at 25 deg C, although this may vary with type and manufacturer.

Practical illuminators may have from 5 or 6 to as many as 100 or more LEDs, depending on output required. Operating voltages are commonly 12V DC and 24V AC. The 24V AC illuminators have built in rectifier and filtering circuits to avoid 120 Hz and 60 Hz hum bars in the camera video. This effect is often seen in video images illuminated by light sources run from AC lines. These illuminators are usually made up of one or more series connected LED strings, The total string voltage is the sum of the voltage drops across the LEDs in that string. Typical IR LEDs run around 1.3 to 1.7 volts, depending on LED current (typically 20 to 30 mA). Consult the manufacturers data sheets for the LED being used, for the exact voltage. This will vary with temperature, being a little lower at high temperatures. As an example, 1.5 volts will be assumed. As the LED current increases sharply with applied voltage, some form of current limiting should be used. This can be a series resistor or a current regulator circuit. The simplest approach is a series resistor, which is often adequate when LEDs are driven by a constant voltage source. The ratio of voltage drop across the series resistor to the LED string voltage should be as high as possible to maintain a constant current, but this gets inefficient for applications where a large number of LEDs must be powered. Pretty good results can be had with 0.5 volt drop for each LED. Therefore a series string of 6 LEDs running 30 mA current ( 9 volts total at 1.5 volts per LED ) can be powered from a 12V source with reasonable efficiency, by using a resistor which drops the remaining 3 volts (12V - 9V) at 30 mA, or a 100 ohm resistor. The power dissipation would be 90 milliwatts, so a 1/4 watt or 1/2 watt resistor would be large enough. Since the entire string dissipates 360 mW and if 3 strings were to be used, this would be over 1 watt dissipation. This can cause a large temperature rise in a small, tight package and must be considered in packaging.

Two illuminators we made up are shown in the photo with views of the LED assemblies and PC boards. The PC boards shown have 40 and 96 LEDs. Layout is not critical and are made to suit individual application. The 96 LED unit operates from 12V DC and is made to fit an electrical box. The round one has 40 LEDs and is made for mounting in a round housing made from 2 inch PVC pipe fittings. A schematic is shown for the 24v AC operated 40 LED round unit shown in our photos. We used Fairchild QED 123 and 223 LEDs (40 and 20 degree 880 nM) and QED234 940 nM LEDs for these illuminators, but you can substitute almost any other type you may have. These illuminators are configured for 12V DC and 24V AC operation respectively, two voltages commonly used to run security cameras. The security cameras used in this application are Sony monochrome black and white (B/W) types. While color is nice, it has no meaning for infrared work as the human eye is blind to infrared (IR). Black and white cameras are more sensitive to low light levels and do not generally have IR blocking filters installed in them, as no color correction is needed. Color cameras often have IR blocking filters ("Hot Mirrors") installed in or in front of the CCD to block IR light that may distort color fidelity, and are generally somewhat less sensitive to light than B/W cameras. Sensitive color cameras generally go down to 0.1 lux with an f/1.4 lens, while B/W cameras can easily achieve .001 lux or better, enabling operation in moonlight. The greater the camera sensitivity the less IR radiation is needed for a given range or coverage area. These illuminators might also prove useful for surveillance photography with infrared digital cameras. In addition, LEDs in the visible range may be substituted if you want to make a spotlight with LEDs, taking note that the forward voltage of visible LEDs is somewhat higher than IR LEDs. With a suitable Infrared Transmitter these illuminators can be used in an Infrared communications system

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