In these colorimeters the spectral responses are directly proportional to the colorimetric functions of the standard CIE observer.
Most of the tools of this type in commerce today have a satisfactory precision. The measurement happens in a such short time, that is possible to perform numerous measurements in the course of the day. Nevertheless, there are few precise tools. That is to say, the trichromatic components obtained do not always agree with the calculations on the given spectrofotometers. These tools should only be used for the appropriate applications.
The trichromatic component colorimeters use filters, photocells and fotomultipliers. The responses that are obtained across the entire visible spectrum are proportional to the colorimetric functions X, Y, and Z of the standard CIE observer.
The radiation reflected by the sample is dispersed by monochrometers. On the plane on which the spectrum is shaped three templates are placed that, reproduce X, Y, Z, so that the spectrum’s response curve of the photocell changes a spectrum of equal energy, thus creating a spectral response curve of the photocell equal to the corresponding colorimetric function.
There are various colorimeters of this type, named “template colorimeters” designed to determine the trichromatic components of bright sources, specially of the fluorescent lights, but also of the transparent objects and reflectors in general. Their precision is very high, especially when the spectrum on which the templates are applied are sufficiently large, for example from 5 to 10 cm. Instead the cut of the masks and the calibration do not result sufficiently precise when the specter is inferior to 3 cm.
A colorimeter “template” is a processing tool is complicated in comparison to the spectrophotometer. It is therefore preferred to use the spectroradiometer or the spectrophotometer coupled with the computer.
There exist also the socalled colorimeters “with filter and photocells”, in which the spectral function response changes can be changed by the colored filtes placed in from of the same photocell. These colorimeters are not as precise as the above. In short, the radiation reflected from the object passes through one of the three filters corresponding to the trichromatic components X, Y, Z and excites the photocell, generating a proportional response for the trichromatic component for this particular object-source combination.
The above mentioned filters are made of layers of colored glass, so that the function of the spectral transmittance resulting from the combination returns the spectral response of the same photocell to the IEC colorimetric function.
The precision of the tool depends on the degree of agreement between the colorimetric functions and the luminancy factor, and the response of the system.
Art 5/7 - Related article: COLORIMETRICS: The Spectrophotometers
I suddetti filtri, in genere, sono costituiti da una combinazione di strati di vetro colorati, di modo che la funzione di trasmittanza spettrale risultante dalla combinazione renda la risposta spettrale della fotocella eguale ad una delle funzioni colorimetriche CIE.
La precisione dello strumento dipende dal grado di accordo fra le funzioni colorimetriche ed il fattore di luminanza, e la risposta del sistema di rivelazione.
ART 5/7 - Articolo correlato: COLORIMETRIA, Gli spettrofotometri.
In theory when a patient has a reduced vision at a distance caused by refraction, an appropriate prescription is required; this is a general rule in the cases of a high visual reduction. When however the reduction is slight, rarely the subject is conscious of his condition and the timely correction should be appraised holding account of the requirements of the patient. Hence the necessity to treat every case like a case to himself: the type of correction depends from the type of rifractivity flaws, from the entity of reduction for the sight to the naked eye, from the type of work type and the temperament of the patient. The problem is emphasized above all in the elderly patients, that present close-by problems; if the examination at distance shows only a slight visual reduction because of an hypermetropy or of an hypermetropic astigmatism, for which two couples of glasses are necessary, one for distant and one for close vision, can put them in embarrassment because they can see well afar and, in fact, they can see very well in daylight. In the normal case one should not insist too much sustaining that the eyesight is reduced and may worsen. etc… but if the subject drives a car he should know that not wearing glasses may have unpleasant consequences. 
In many applications it is useful to refer to relative luminances, resorting to the so-called “colorimetrics by difference”.