A $20 price tag puts a different perspective on things and makes my reply about the Macbeth calibration reference look rather silly, but it all depends on what you want to use it for...
The standard filters in the TR-927 read Status T RGB (wide band gelatin filters: Wratten 25, 61 and 47), ortho and UV (Wratten 18A). Depending on your intended use, it may not be necessary to use a calibrated step wedge. If you did want the highest accuracy you may need to bear in mind that a narrow band instrument (eg TR-924 with Status A and M dichroic filters, and visual) might give a different reading from a wide band instrument, depending on the ‘neutrality’ of the target – so there’s a possible twist to ‘calibrated step wedge’. But it probably doesn’t matter, just something to be wary of if you wanted to measure colour and correlate the readings with another instrument to the second dp.
With the TR-927 the green filter is normally used for reflection calibration, and the ortho for transmission, however there’s no point in using the green filter for calibration if that isn’t a channel you are interested in. The 61 green filter is usually the fastest to fade, and differential fade could throw the other readings off.
As far as the frequency of calibration goes, I thought that it was normal practice to zero every channel every session, for which you need a reflection reference but not a transmission reference, and calibrate every few days. If the instrument has changed hands, the calibration pots may have tempted idle fingers. I calibrate every session because it is so easy, though I suspect that I’ll soon fall to the charms of a modern, low cost spectrophotometer – the reason that there are all these cheap densitometers on the market?
PS Claire – Me? I ain’t been up to nothin’, honest.