Tuesday, February 1, 2011

LED Array Illuminator

A long range infrared illuminator can be made using many LEDs

There are times when the small infrared LED ring built into a security camera will not cover the range or field of view you require, so you will need to find another invisible light source. Some large infrared illuminators use powerful incandescent light sources that are passed through an infrared pass filter, causing only the infrared component of the light to come through the filter. These types of infrared illuminators create intense heat due to the fact that the white light source must be fully enclosed and burn the unwanted light energy off as radiated heat. Because of this intense heat, incandescent filtered illuminators cannot be used indoors and may not be suitable for many outdoor installations.

The good news is that LEDs can be used to create a very powerful infrared illumination system if you use enough of them. Ok, you need a lot of them, but these days they can be purchased for only pennies a piece if ordered in quantities of hundreds or more. The bad news is that you will need to do a lot of soldering, even on a small array of 16 by 16 LEDs, which will have more than 512 connection points. Of course, many circuit board houses offer proto service and you could have a very large LED array circuit board made for under $100 if you shop around. If you are patient and like to solder, then any size array can be made on some perforated circuit board, resulting in a very high power illumination system that will only cost you 1/10th of what a manufactured unit would cost.

Figure 1 - LEDs purchased in large quantities can often be found at bargain prices
Figure 1 - LEDs purchased in large quantities can often be found at bargain prices

Before you decide on making a huge array that will light up an entire city block, do a little research on bulk LED prices and power requirements because an array will become hungry on both counts. I built two version of the LED array - one using a hand wired perforated board having 13x19 LEDs and a much larger PCB version having 32x48 LEDs. So the smaller LED array has 247 LEDs and the larger array has a whopping 1526 LEDs! Make no mistake - it takes a good chunk of power to crank up 1526 LEDs to their maximum potential, and even at 10 cents per LED, that adds up to $154 just for the LEDs.

Start by calculating how much infrared radiation you will need in order to light your scene. Limitations will likely be the focal range of your camera since details are lost on most security cameras after about 50 feet. This distance is also about as far as an LED can reach, no matter how many you add to the array, so the equation then becomes how wide and how bright do you need the scene? A 20 foot by 20 foot interior room will shine like mid-day with an array of 16x16 LEDs at each corner of the room, but the massive array I built is almost too bright to be used indoors. If your camera will stay in a fixed position, then a single array is best, but for general room illumination, it is better to divide up your LEDs into two or more arrays for even lighting. Think of a 16x16 LED array to be about the same as a typical hand held flashlight for both output power and field of view. My 32x48 array acts more like a 500 watt halogen light source when placed in a small room.

There are several varieties of infrared LEDs, ranging in size, field of view, output power, and effective light color. The most commonly used infrared LEDs output 940 nanometer infrared light which is far beyond the human visual range, and fairly detectable by any non filtered video camera. There are also infrared LEDs available for the 800 to 900 nanometer range. These are even better for use in night vision applications, but there will be slightly detectable red glow as the human eye can faintly detect this band of light. If you have seen an outdoor night vision security camera after dark, then you are probably familiar with this dull red glow. The LEDs shown in Figure 1 are commonly available 940 nanometer types purchased in bulk from an Internet based supplier.

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.