[argyllcms] Re: [Possible bug in Spyd2.c] Inconsistant readings from Spyder3

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 07 Mar 2009 01:28:07 +1100

howdy555@xxxxxxxxx wrote:
Hello again!

As I wrote in the last post I have found a possible bug in Spyd2.c.
Spyder3 has problems with reading a very dark patch and can sometimes
return garbage:
To confirm the bug try reading a completely black patch a few (10?)
times with spotread and you will see that the results are something
like (for X axis):0.2, 0.19,0.2, 0.17 [so far OK], 0.01 [Stupid reading!],
0.2, 0.19 [etc.]. The problem is IMHO caused by too small reading time in 
spyd2.c. When I
added (file: Spyd2.c):

The spyder 3 produces values of exactly 0.0 all the time for black on my CRT.
The manufacturers driver produces the same reading.
Even with your modification (bumping the time up to 32 seconds),
I get XYZ values of exactly 0.0.
[For comparison the i1pro produces XYZ values of 0.048 0.051 0.064,
and the Spyder 2 values of 0.035 0.042 0.043 ]

Given that the manufacturer's driver integrates for 2 seconds, while
the Argyll driver integrates for up to 6 seconds total, my suspicion
is that this is simply near the quantization limit for this particular
device, which seems to be oriented mainly for LCDs. Yes, you can raise
the integration time some more, but this makes it very, very slow,
(making things like adjusting the displays controls very difficult),
and doesn't completely solve the problem (there will still be some lower
level of light where it behaves the same way as you are seeing on your

Graeme Gill.

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