Tech Tips for IR illuminated CCD cameras - this is the companion page for pir-remote.htm.

The camera(s) I'm using are from Allthings. They are an Australian company selling a range of CCD cameras etc. They also have applications notes at their site. Some of the data here come from those notes. I have no relationship with "Allthings" other than as a customer.

Modern PCB video cameras are excellent value for money. However when I tried to use one to catch some thieves who have been stealing petrol for my car at night - I found there are traps for beginers. My goal was to automatically video noctural visits to my car - with good enough image quality to identify the thieves and also be useful as evidence in court. A friend loaned me a camera - so I could see if it was what I needed before buying my own. The camera specs say it operates in low light levels - down to 0.05 lux. Pretty impressive I thought - the street lights across the road should just about give enough light for the camera to see by. I rigged the camera up to my VCR and pointed it at my car. When night fell the picture went black - a few glints of chrome was all that showed up on the TV screen. I rigged up 20 IR (infra-red) LEDs (light emitting diodes) and there was some improvement but not enough to meet my goals. So what went wrong?

Camera sensitivity - After many hours searching the internet I still don't know how sensitivity is defined. The definition of "lux" is ok but what is response of the camera at the specified minimum light level? Is it a "full well response", a signal just above the noise floor or somewhere in between? I would presume the illumination is monochromatic light at the peak response wavelength (530 nm). It's not that I care too much what definition is used - what I want to know is do all brands us the same standard. If not then comparing different brands could be very miss-leading.

Lens power - The sensitivety of the camera will vary with the light gathering power of the lens - the "F" number. The sony camera I'm using will work in 0.05 lux with an F1.2 lens. The pinhole len the camera came with is most likely F3.8. The "F" stop relates to the diameter of the appature compared to the focal lenght of the lens. The amount of light passing through the lens is determined by the square of the number - this is why F-number are usual arranged in steps of root-2 (1.414aprox) - with each step doubling the exposure. The lower F numbers pass more light - F1.4 passes aprox twice what an F2 does. Anyway an F1.2 lens passes slightly over 10 times the light a F3.8 does - this goes a long way to explaining the poor results I got. Our 0.05 lux camera is really 0.5 lux. The sony camera (and many others) take two different styles of lens - the "pinhole lens" and the "board lens". The term pinholes lens is confusing to people who know what a "true" pinhole lens is. A small hole will act like a crude lens but these pin-hole lenses are usually a small single element glass lens in the order of 1mm across. They are very usefull if you want to hide a camera behind a small hole but they are not good for low light levels. The board lenses are larger diameter, multi-element lens with better F numbers - but F1.2 is still uncommon. I bought a 8mm F2.0. An F2.0 is a 3.6 times improvement over F3.8. Board lenses tend to have better image quatily as well. Lower F numbers reduce the depth of field making focusing more critical but this is unlikely to be a problem with short focal length lenses.

IR sensitivity etc. CCD cameras are not designed to be IR sensitive - nature just happened to make silicon's light sensitivity different from our eye's receptors. This IR sensitivity is often unwanted and reduced by means of IR-cut filters. While CCD cameras can see IR they don't do it very well and they only see near-IR - that is it portion of the IR spectrum close to the wavelenght of visible (red) light. The camera can't see far-IR at all, they can't see body heat like thermal imagers or PIR (passive IR) detectors can.
The Relative Response (Spectral Sensitivity Characteristic) of a typical Silicon CCD Image Sensor (SONY) is approximately: 0.5 @ 400 nm, 1.0 @ 530 nm, 0.35 @ 730 nm, 0.025 @ 950 nm
This is a bugger as lighting the crime scene with IR at 940 nm will require close to 40 times the light intensity as you would if you could use 530 nm (green). Using IR with shorter wavelengths improves this but then the light source may become visible. Typically the light source is an array of IR LEDs and all but the longest wavelength IR-LEDs are visible in total darkness - just a dull red glow - most people probably wouldn't notice - but then again they may. IR lights can also be made by placing an IR-pass filter over an incadesent lamp. I bought some FIL-IR670 filter material but it passed too much visible light to be useful for this purpose. IR illuminator kits are reasonably priced but I think they look a bit obvious - there is room for some personal creativity and sneakiness in building your own design. Buying a kit may still be the cheapest way to buy the LEDs.

Over-exposure damage. My camera has automatic electronic exposure control which can vary the exposure by a factor of 2000:1 or so. This still isn't enough to cope with daylight. In my setup the camera is viewing an outdoor area so this is a problem. The Allthings app notes state that long term over exposure will damage the CCD - but how much over-exposure? and how long? It also likely that some wavelegths do more damage than others. One solution is to fit the camera with and auto-iris but this becomes expensive and most likely makes the camera bulky. A home made shutter is also a possibly - I did design a cheap shutter once for a optical instrument I worked on. It was based on a long travel solenoid ($25 approx) it works well but does tend to sound like a rat trap. It had 12mm travel which is much more than needed here. I think it's do-able but not on my agenda. What I am doing is making this an IR-only system. By placing some FIL-IR670 material in front of the lens I'm blocking almost all the visible light, I'd expect this to reduce over-exposure damage. It should also give a slight sharper image. IR light has a slightly different focal point to visible light so a system which uses both visible and IR will never be quite as sharp as a IR-only or visible-only system. The drawback is I can't take advantage of existing visible lights - such as the street lights. The filter also hides the camera lens from sight and makes it weather proof.

More to be added when I test the new camera....