Thoughts on the Philips ToUcam for imaging Mars.
Goto Japanese Edition 火星撮影に関する Philips ToUcam の考察 (日本語版)
Damian Peach & Tan Wei Leong.
Thoughts on the Philips ToUcam for imaging Mars.(English)
Spectral Sensitivity Characteristics of ICX098 CCD chip used in the Philips ToUcam.
As can be seen the spectral response of this CCD when used for producing RGB images from splitting
from the master colour image will not produce data of the same quality as of the images had been
obtained through the proper filters.
The problem lies in that, the response of the chip in Blue and green overlap a great deal at ~475nm.
Also, a R leak occurs going far back into the 450nm range.
Note that simply using an IR block filter will only help a little to cut the R end of the response down,
but the problem still lies in G-B overlap, and the red leak.
This in summary means that the Blue image data is “polluted” with G and R (which is why the surface
features remain visible, even when an IR block filter is used.) Also, the G image data is slightly
polluted by the Red leak.
In further reviewing the documentation, the way in which the CCD reads the data out also causes problems,
as 2 x as much G signal is generated than R or B.
We present one possible solution to this problem below.
Spectral Sensitivity Characteristics of True Technology CMY filters
Looking at the spectral characteristics of the TT CMY filters, we can “pre-filter” the ICX098 CCD chip
with the magenta filter including a suitable IR rejection filter.
This will get rid of the band from 500nm to 600nm, and block both the G leak into Blue, and Red leak.
Signal lost in R and B would be kept to a minimum.
We can reconstruct a synthetic G from the R and B (with a ratio that can be easily worked out).
Since the G frame normally does not contain useful data for Mars, this will help to get better B and R
frames using the ToUcam with very minimum modification and hopefully will encourage more submitted
images with useful data.
Our final thoughts however, are perhaps while the ToUcam is a superb choice for producing
high resolution colour imagery, perhaps the cooled CCD systems with proper filters still very much have
the edge when one wishes to produce filtered imagery of Mars.
Tan Wei Leong.