Julia Claxton wrote: > The above link will take > you to a page which explains the CRI [colour rendering index] used to rate > their lighting. The higher the quality of light output compared to natural > daylight, the higher the rating on a scale of 0-100.
Julia and all, CRI is a little bit more complex than that (and IMHO not that useful after all). Please don't be put off by this I used to think the way you do too: The Colour rendering index expresses the degree to which 8 special pastel and 9 supplementary colours appear natural under a particular light source. The light source to be evaluated is compared to a reference source at a specified colour temperature. Lamps with a colour temperature below 5000 Kelvin are compared with Tungsten (tungsten being awarded a CRI of 100). For light sources above 5000K the reference source is the daylight phase (the D 65, 70 etc.) that matches it's correlated colour temp. The colour rendering index is based on arbitrary reference sources. This does NOT mean, however, that a reference source has good colour rendering capabilities, it's just the way it is. (We can probably all agree that tungsten/CRI 100 isn't ideal for viewing colours in). CRI is the average performance of a specific light source compared to the reference. Better characteristics in some colours can be achieved by sacrificing others which might leave you with a high CRI number but inferior characteristics. Two lamps with similar correlated colour (Kelvin) and same CRI may differ considerably when comparing certain colours. Horizon level daylight and a tungsten bulb both has a CRI of 100 because these are compared to themselves, where a xenon lamp at roughly 5900 K "only" scores 94, but then this is compared to D65... CRI is only an indication of the abilities of a light source. It only makes sense when you know and understand the deficiencies and limitations of the light source in question, and the CRI system. And you only have an idea of the lamp deficiencies once you've measured the light with a spectrophotometer (or a spectroradiometer) and seen the spectral distribution curves. Now there's a reason to purchase an Eye One (Eye one Share can measure and plot light/SPD curves) before spending all that money on �5.99 fluorescent tubes <G> Best Regards, Thomas Holm / Pixl Aps - Photographer, Educator, Colour Management Consultant & Seminar speaker - Remote Profiling Service (Output ICC profiles) - www.pixl.dk � Email: th[AT]pixl.dk -- =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
