IR illumination 101

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IR illumination 101

Post by phoenix » 18 Feb 2014, 16:46

IR illumination 101
1. Wavelengths - visible light runs from around 400 (violet) to 760(deep red) nanometers (nm) IR is anything longer than that and the two most common wavelengths used for digital NV are 850 and 940nm.
850 gives longer range, because cameras are about 2-3 times more sensitive to light at that wavelength then they are at 940nm. 940nm is more covert than 850nm and is generally regarded as being much less visible to potential quarry than 850nm. Having said that, the majority of NV builds use 850nm illuminators

2. Sources of IR -
a. White (halogen) light and filter. Power hungry and inefficient because IR is only a small part of the total light produced by a halogen lamp and the filter has to block (and waste) most of the light which is being produced. Typically uses 12volt 60 watt lamps plus a dark coloured filter - the dinosaur of IR sources, but still used by some of those who have never had the good fortune to become members of this forum. Note - White LEDs and a coloured filter don't work because white LEDs don't produce any IR.

b. Infra red lasers. Small and very efficient. Still popular with tubed NV users. Now looked upon with disdain by many digital NV users because most of them produce a "dirty" beam with lots of artifacts and variations in brightness. There are also serious safety issues with IR lasers in that eyesight damge can easily occur if the laser beam is looked at directly or enters the eye via a reflection from a smooth surface.

c. IR LEDs (light emitting diodes) - currently the preferred source of IR for digital NV work, and one IR LED in particular, the Osram Oslon (part no SFH4715S). With the correct optical arrangement this LED will easily illuminate potential targets at more than 300 metres, yet only consume 3 watts of electrical power

3. Complete IR illuminators - The most common arrangement is using an aspheric lens mounted in front of the LED so that a tight beam of light is produced. Adjusting the distance from the lens to the LED controls beam width. At present, several fourum members manufacture and sell IR illuminators based around standard flashlight types.
For close range work the 501b flashlight fitted with an 850nm oslon and an aspheric lens is an excellent choice. Forum member "some bloke" sells these.
For longer range work, the T20 flashlight, again using an 850nm oslon but with a somewhat larger aspheric lens is very popular. Forum members "Sika Stag" and "Marky 610" sell these.
In general, increasing the size of the aspheric lens will increase the usable range of the illuminator and aspheric lenses up to 78mm diameter have been used with success, although 66mm is the most common of the larger sizes.
Hope that's enough to get you started in the wonderful world of IR