Digital BW, The Print

Roger L Sopher <rlsopher@c...> wrote:
> In analytical chemistry the difference between a colorimeter and a 
> spectrophotometer is basically in the method of producing the 
> "monochromatic" light source used for measurement.  A colorimeter 
> typically uses filters although some are now using arrays of LED's and 
> produce a narrow band of wavelengths rather than strictly monochromatic 
> wave lengths.  A spectrophotometer is capable of producing a spectrum 
> of distinct  monochromatic wavelengths and typically uses a diffusion 
> grating and a series of pre-filters .

Fascinating. I guess an absorbtion spectrometer bears more resemblance to a negative densitometer than a print densitometer or analyzer. 

> Some older spectrophotometers 
> used prisms rather than gratings. The X-Rite 810 etc would fit the 
> category of colorimeter.

All the filtering is done on the received light, typically a diffraction grating in front of a small linear CCD. This allows the spectrophotometer to do double duty, measuring the spectra of emissive targets such as CRT and LCD monitors, or the spectrum of the light in one's print viewing environment. For print densitometry, the spectrophotometer contains a small white light source (which typically has a spectrum that includes ultraviolet, because papers have flourescent brightners added). 

> Calling them densitometers simply means they 
> can do the conversion from percent transmission, %T,  or percent 
> reflectance,  %R to optical density, O.D., D,  (mathematically, D = 2 - 
> log10  % T)  internally so that they can  output  a linear rather than 
> a logarithmic result.

Yup. Except that the convention is that color densitometers have three narrowband filters, colorimeters have three wide band colormetric filters, and spectrophotometers have an awful lot of narrowband filters or a diffraction grating and a fairly high resolution CCD to read it. It's not a great convention, but it's all we've got...