Fundaments of Photometry and Reflectance

What is Color?


Visual perception of color is relative and subjective. The color we see is a result of

the wavelengths that are reflected back to our eyes: 

  White light is made up of many different wavelengths of light, and the whole light- color-spectrum form a rainbow.


 Any object present in our environment can have a color if its surface reflects or transmits light on the visible wavelength spectrum 360 - 720nm.


Light sent to an object will be reflected or absorbed depending on surface

characteristics. For example, green paper will absorb most of the reddish and bluish

part of the spectrum while reflecting the greenish part of the spectrum, making it appear greenish to the observer.


White objects appear white because they reflect all wavelengths of light. Black objects absorb all wavelengths of light so no light is reflected.


What is Colorimetry?


Colorimetry is the science and technology used to quantify and describe physically the human color perception. It is an effective method for testing water quality and used by Palintest in the visual or photometer product platforms.


This process usually involves adding a chemical reagent to a water sample. The color, once developed, will be evaluated in reference to known color reference (either printed or stored in a photometer) to determine a concentration.


How is Light Measured?

Light is measured by firstly activating a light source. The light emitted is passed

through the sample and the transmitted component is measured on a photo-detector.


.Any light which is not reflected or transmitted is absorbed by the sample.


How Does the Photometer Work?


Photometers are digital colorimeters used to remove any subjectivity that results from color assessment with the human eye.


Photometers measure parameters with improved accuracy in comparison to visual colorimeters such as test strips, test blocks or comparators.


All photometers consist of the following elements:


   A light source

   Filters to apply selectivity with respect to wavelength

   A sample compartment for the test solution

   A photo-detector to measure light and produce an electronic signal leading to

a digital display.


 By comparing the amount of transmitted light through the untreated sample (a blank) and the amount of transmitted light through the test sample, the concentration of the parameter tested can be determined.


Photometers calculate the measured concentration and then display the test results directly in mg/L or ppm.


How Does a Wavelength Filter work?


The light transmission at a particular wavelength ensures sensitivity to a particular chemical.


A wavelength filter allows only the wavelengths of interest to be separated from the overall spectrum.


Each test carried out with a Palintest photometer will automatically select the wavelength to be used for each test.


How Does a Photo-detector work?


A photo-detector is a sensor which responds to light.


In a photometer, the light present at the detector is converted into an electrical signal.


The instrument has been calibrated so that the intensity of the signal generated is

measured by the electronic circuit in the photometer and directly translated in a concentration.


How Does a Photometer Measure Reflectance?


Reflectance is the amount of light reflected by a surface relative to the light emitted.


A reflectance photometer can measure the amount of light reflected from a colour as a function of wavelength. In this process, white light is sent to the coloured sample and the reflected light is measured by a RGB colour sensor.


What is the RGB Colour Space?


An RGB colour sensor relates instrument response to the RGB colour space model. This is an additive colour model in which red, green, and blue light are added together to produce a broad array of colors.


The tristimulus values in terms of International Commission on

Illumination in English (CIE) X, Y and Z values of the unknown colour

can be calculated from the R, G and B values, which allow us to define

colours in three dimensions rather than the two associated with transmittance photometry.


Advantages of Photometry


   Very low concentrations of species can be determined accurately


   Analysis is relatively free from interferences


   Optical sensors can be more reliable than electrodes


   Calibration curves are relatively stable with respect to drift


Disadvantages of Photometry


   Higher concentrations are difficult to achieve with accuracy so respect must be

given to the stated range of the test method


   Optical measurements rely on the solution being completely transparent. Solids

in the test tube must be handled carefully either allowed to settle or filtered


   Background colour of sample may interfere so the blank correction process is

very important. The image in 'How Does a Photometer Work' shows the generic process of a photometric method in terms of time and colour development.


   Unless the blank (A0) is corrected, the result will be incorrect and also the time

stated in the operating method must be observed for optimum accuracy.

Aldo Zaffalon
Aldo Zaffalon


Water treatment automation expert, with over 25 years of experience in industrial water treatment automation, marketing, sales and engineering.

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