IR Blocking Filter

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This filter has a 90% pass band from 370nm to 575nm. The plot is also log scale which tends to make it look more curved than it is when compared to a linear plot. It is cutting a  bit more red light than the interference type filter . However, most CCD's are more sensitive in the red area anyway.  The QC B/W's TC255 CCD has higher red sensitivity which would balance well with this filter (See Below).  


Matching the filter to the CCD

The filter is not the only element in the optical system. The CCD in your web cam has a frequency response of it's own. and both of these play together to give the final result. Typical CCD response curves are given in Astronomical Image Processing, by Berry & Burnell.

TC255 CCD Responsivity Curve. Typical of most CCD's (Note: LOG scale also). 

A flat filter response does not give you a flat response on the image since the CCD does not have a flat response either. I investigated this further to see how the filter would perform as a system with the TC255 CCD. I entered the filter curves for both the F-UWIR and a nominal interference filter into a spreadsheet. (Log scale version) I then multiplied the effect of the filter transmission and the CCD sensitivity across the band. If light started with a value of 1 then this is how much the CCD would pickup at different frequencies. The results are given in this graph.  (Log scale version) You can see that the results are fairly close. The F-UWIR appears to give you more in the blue/violet end but their is so little ( only about 9% ) light actually doing anything here that the gain is insignificant. While this looks good it is important to also match the system to the objective. After all we are only doing this because our refractor doesn't focus all the light in the same place. So how much does it focus.

Refractor Focus Bandwidth

We would all like a Refractor to focus all light on the same point. Fact is they can't, even the best of them are compromises with respect to chromatic aberration. More expensive refractors hopefully give you a wider band of colour that is focused adequately and better off axis performance. Most of use probably haven't got the best refractor money could by or we wouldn't be messing with webcams. So what is a typical range for your average refractor. Rutten & van Venrooij in Telescope Optics point out that a refractor designer should aim to cover the range that is most sensitive to the eye. At least then it will look good visually. Here is the plot for the average eyeball:

So from this we could assume that your average refractor is probable well corrected between 450 and 625nm and a better one maybe 400-675 nm. Looking back at out imaging system plot we see that the 350-450 region probably isn't going to be well corrected for colour but still won't be a problem because our imaging system sensitivity here is very low (less than 9%). However, at the 650 end we are getting close to the maximum amount of light getting captured by the CCD. Note: for interference filter this is the maximum of 30% reception, for the F-UWIR this region is below maximum at 24% reception. The 650 nm region could well be a problem area.

Fortunately Rutten & van Venrooij in Telescope Optics  also include a typical visually corrected doublet plot in their book. As you can see the good news is that the curve is very polite at the 650nm end, curing away gradually and the design point is correcting for 650nm. Lucky for us that we have very little reception at the violet end as the curve is  rather nasty down there.


Steven Mogg