Why would you suggest I need $500 systems to get round-about accuracy tomeasure daylight?
You haven't stated what you are trying to do.
Television sets more-or-less universally come with sub-$10 ambient light measuring systems that are sufficiently accurate under the two main sources of (well understood) light: daylight and incandescent light. Isn't it because the sources of light are so invariant (relatively) that you only need the most rudimentary means of measurement?
This is a mailing list for ArgyllCMS, which implies that color accuracy is of some interest. For most calibration and profiling, people like to see accuracy in the 1-4 delta E range. If you don't care if your accuracy is +/- 20 Delta E, then you are after something unusual for this list. I'm not sure what sort of systems you are referring to in Televisions. There are some cheap colorimeter modules about that could conceivably be used for very roughly estimating daylight color temperature. The difference between them and a DTP94 is that they will (may ?) have been calibrated against daylight and other typical domestic illuminants, whereas the DTP94 has not.
Afterall the quality of daylight does change with cloud cover -- but in a predictable way.
To any degree of accuracy, daylight is not "all the same". It varies with season and latitude and weather. Add in other random light spectra (such as incandescent, fluorescent and LED lamps), and no colorimeter is going to give anything other than an educated guess. If you are after more than this, then a spectrometer is needed. Graeme Gill.