Digital BW, The Print

c1asia wrote:
> can you explain IR high-pass and low-pass filters to me? i'm not
> familiar with IR.  thanks.
>
>   
OK, it's easiest to start with a bit of basic physics. Light is part of 
the electromagnetic spectrum, along with radio waves, microwaves, X-rays 
and gamma rays. Like all the other forms of electromagnetic radiation, 
light (visible or otherwise) has a characteristic frequency. Radio 
receivers tune to a particular frequency by filtering out other 
frequencies that aren't relevant -- similarly, the human eye perceives 
colour with three sets of colour receptors, one tuned for red, another 
tuned for green and another for blue. In terms of frequency, red is the 
lowest that is visible, then green, with blue being the highest. 
Infrared light isn't visible because the frequency is too low for it to 
be detected by any of the three sets of receptors -- the name basically 
literally means 'lower frequency than red'. In photography, we generally 
deal with what is known as short wavelength IR, also known as near-IR. 
This is close to visible light in frequency, and should not be confused 
with 'seeing heat' -- the thermal imager footage often seen on TV is 
actually much lower frequency (longer wavelength) than the kind of light 
normally dealt with in infrared photography.

Conventional film is only significantly sensitive to visible light, so 
no special technology is needed in cameras or lenses in order to capture 
visible light images. However, silicon imagers, including both the CCD- 
and CMOS-based sensors used in digital cameras, are actually sensitive 
well into the infrared as well as to visible light. This is a 
consequence of the physics involved, it's not something that is 
generally deliberately engineered in. Normally, this infrared 
sensitivity isn't something you want, because it can tend to cause weird 
colour shifts, particularly in strong daylight (where there is a *lot* 
of infrared from the sun). As a consequence, nearly all digital cameras 
contain an infrared high pass filter -- literally, this is a piece of 
specially treated glass that typically looks like it has a slight 
cyan-blue cast to the naked eye, which blocks infrared very strongly, 
whilst passing through visible light more or less unaltered. As I 
mentioned in my previous post, the Better Light doesn't have a high pass 
filter on its sensor, so you have to use an actual filter attached to 
the lens. This means that you can take this off and replace it with an 
infrared low pass filter that blocks visible light and passes infrared 
-- this is key to getting good sharpness in infrared, because lenses 
have a slight focus shift toward the IR, so it is difficult to get both 
IR and visible light in focus at the same time.

Hope this helps,
Sarah