hi kai,

there is no need intensity to be constant for all the instrument life. my instrument works on beers law which states

k = abc

K = constant
c = concentration of the subject in a coloured solution
a = absorvance of the solution
b = path length of the light trough solution(kept 1 cm)

the equation simplifies to

k = ac

c ~ 1/a

for a constant K

c1/c2=a2/a1

The instrument is called a colorimeter. my instrument is automatic colorimeter. anyway , the wavelenth of the light being passed is kept such a value that its color is complementry to the color of solution. thats because it has highest absorbence for it , causing less error in measurement.

The blood being tested for some biochemical like glucose or cholesterol is sampled from the patient and revolved in a high RPM centrifuge so that serum and RBC and other courpsals are separated. this serum has the biochemical under test. this is mixed with some other reagents/enzymes to produce coloured solution. this colour is direct measure of the concentration.

now to run a test my instrument asks for the blank solution which has no concentration of the biochemical, and the light beam filtered with certain filter (described above) is passed throgh it and intensity is measured(Ib). again it asks for the standard solution. this coloured solution has the known concentration of the biochemical, again intensity is measured(say Is). now the sample is tested and intensity is measured(say It).

abs of the standard solution= a2= Is-Ib
abs of the sample solution = a1= It-Ib

as c2 is known

then c1 = a2/a1*c2

after this (blanking and standerization) n number of tests can be repeated without testing the blank and standard. during this the intensity of light should not change

This also explains spectral requirements of the light source.


abhishek