This article illustrates the principle and working of Colorimeter and Photometer and how absorbance, transmittance and light intensity can be measured.

1. Colorimeter & Photometer
ER. FARUK BIN POYEN
ASSISTANT PROFESSOR
DEPT. OF AEIE, UIT, BU
faruk.poyen@gmail.com

2. Contents:
 Colorimeter – Definition.
 Colorimeter Theory
 Colorimeter Schematic Diagram & Equipment Diagram
 Parts of Colorimeter
 Principle Components
 Pros & Cons of Colorimeter
 Standard Photometer
 Types of Photometer
 Characteristics of a Good Photometer
 Single Beam Filter Photometer – Working Principle
 Double Beam Filter Photometer – Working Principle
 Difference between Spectrophotometer & Colorimeter
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3. Colorimeter:
 A colorimeter is a device used in colorimetry.
 In scientific fields the word generally refers to the device that measures the absorbance
of particular wavelengths of light by a specific solution.
 This device is commonly used to determine the concentration of a known solute in a
given solution by the application of the Beer-Lambert law.
 Beer Lambert Law states that the concentration of a solute is proportional to the
absorbance.
 It is invented by Louis Jules Duboscq in 1870.
 It involves quantitative estimation of colour.
 The colour of light is the function of its wavelength.
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4. Colorimeter: Theory
 As a monochromatic light is made to pass through a coloured solution, some specific
wavelengths are absorbed which are related to the colour intensity.
 The amount of absorption by a colour solution is in accordance of Beer Lambert Law.
 The intensity of colour is directly proportional to the concentration of the compound.
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5. Colorimeter: Basic Schematic
 An Op-amp based amplifier is used to amplify the signal before putting on meter.
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6. Parts of a Colorimeter:
 The essential parts of a colorimeter are:
1. A light source (often an ordinary low-voltage filament lamp);
2. An adjustable aperture;
3. A set of colored filters;
4. A cuvette to hold the working solution;
5. A detector (usually a photoresistor) to measure the transmitted light;
6. A meter to display the output from the detector.
 In addition, there may be:
1. A voltage regulator, to protect the instrument from fluctuations in mains voltage;
2. A second light path, cuvette and detector. This enables comparison between the
working solution and a "blank", consisting of pure solvent, to improve accuracy.
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7. Colorimeter – Equipment Diagram
 (1) Wavelength selection, (2) Printer button, (3) Concentration factor adjustment, (4) UV
mode selector (Deuterium lamp), (5) Readout, (6) Sample compartment, (7) Zero
control (100% T), (8) Sensitivity switch, (9)ON/OFF switch.
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8. Principle Components:
 Filters:-
Changeable [Filter (optics)|optics filters] are used in the colorimeter to select the wavelength
which the solute absorbs the most, in order to maximize accuracy. The usual wavelength
range is from 400 to 700 [nanometer] (nm). If it is necessary to operate in the [ultraviolet]
range then some modifications to the colorimeter are needed. In modern colorimeters the
filament lamp and filters may be replaced by several (light-emitting diode) of different
colors.
 Output
The output from a colorimeter may be displayed by an analogue or digital meter and may be
shown as transmittance (a linear scale from 0-100%) or as absorbance (a logarithmic scale
from zero to infinity). The useful range of the absorbance scale is from 0-2 but it is desirable
to keep within the range 0-1 because, above 1, the results become unreliable due to
scattering of light.
 In addition, the output may be sent to a chart recorder, data logger, or computer.
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9. Principle Components:
 Cuvettes
In a manual colorimeter the cuvettes are inserted and removed by hand. An automated
colorimeter (as used in an AutoAnalyzer) is fitted with a flow cell through which solution
flows continuously.
 Cuvettes are rectangular or square cell made of optical glass (quartz) to hold the sample.
 Optical path length is 1 cm.
 Cuvette must be transparent, clean, devoid of scratches and no bubble on the inner side.
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10. Concentration – Absorption - Transmittance:
 Wavelength between 400 nm and 700 nm form the visible spectrum.
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11. Pros & Cons of Colorimeter:
 Pros:
1. Inexpensive.
2. Well suited for quantitative analysis of coloured compounds.
3. Easily transportable.
 Cons:
1. Not suited for colourless compounds.
2. It does not work with UV and IR spectrum.
3. Specific wavelength can not be set.
4. Interfering substances from similar colours can produce errors.
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12. Standard Photometer:
 Photometer is a photometric quantity detector;
 It consists of a silicon photodiode, an aperture, and in some cases a diffuser and V(λ)
correction filter.
 This V(λ) correction filter must follow the total spectral responsivity of the photometer
(photodiode, filter, diffuser) to the V(λ) function.
 Again the photometer head does not need to have cosine correction.
 It is because this photometer is generally used with an incandescent standard lamp.
 This lamp is placed on the optical axis of the photometer at a sufficient distance to
deliver normal incident light with small divergence angle.
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13. Standard Photometer:
 For achieving the accuracy of the photometer responsivity, the reference plane is
accurately defined.
 The reference plane is the plane where the inverse square law is to be defined.
 So this reference plane is not erroneously defined to break the accuracy of inverse
square law based on detection of the characteristics of the photometer.
 A photometer is a device to measure the intensity of light and strength of
electromagnetic radiation in the region from UV to IR spectrum.
 Photometers detect the light using photo resistors, photodiodes, and photomultipliers.
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14. Types of Photometer:
 Here two types of photometer are shown below.
 The figure shows that a diffuser is just placed between the aperture and the V(λ)
correction filter.
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15. Non Diffuser Type Photometer
 It is a standard photometer without diffuser.
 It is often used by the national laboratories to realize and maintain the Illuminance unit.
 By using collimated monochromator output beam this type of photometer is used to
measure the spectral responsivity.
 It exhibits narrow acceptance angle.
 Narrow acceptance angle is advantageous for measurement as it blocks the stray light
from the ambient.
 But disadvantage is that it uses the large size lamp at shorter distance.
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16. Diffuser Type Photometer
 It is a standard photometer equipped with a diffuser.
 It is very commonly used.
 This diffuser is used for cosine correction.
 The diffuser provides flat reference plane precisely.
 Illuminance meter having a dome-shaped diffuser is not adequate for standard
photometers.
 This diffuser is not subjected to the UV ray to be degraded.
 It exhibits large acceptance angle.
 But it is less subject to the errors due to the large sized lamp at shorter distance.
 It is more subject to the stray light effects due the large acceptance angle.
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17. Characteristics of a Good Photometer:
 Relative spectral responsivity must be matched with V(λ) function. Generally most of the
photometer responsivity do not match with the V(λ) function. But good photometer needs to
be with very small error.
 Temperature dependence is a factor of the photometer responsivity. Due to temperature change
the photodiode changes its responsivity. So temperature correction procedure should be
followed.
 Linearity of the photometer sensitivity depends on the silicon photo diode sensitivity. So high
quality photo diode must be provided with the photometer.
 Long term stability should be fulfilled by the photometer. The response of the photometer
should not be changed within a short period of time.
 Calibration should be provided with respect to the reference plane. This calibration must be
done at 25°C i.e. room temperature. The distance, room temperature and the reference plane
should be recorded.
 Reduction of the stray light must be done with care to minimize the error.
 High Illuminance level must be maintained during measurement.
 Maintenance should be done periodically to make the photometer safe from dust and moisture.
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18. Single Beam Filter Photometer – Working Principle
 Single beam filter photometer consists of a source of light (tungsten filament lamp), lens
to make the light beam parallel, filter of wavelength selection, cuvette with sample holder
for keeping the solution to be analyzed , detector (photocell) and reading device
(galvanometer or micro-ammeter).
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19. Single Beam Filter Photometer – Working Principle
 The tungsten filament lamp gives the light radiation.
 This light is incident on the lens which makes it a parallel beam of light.
 This parallel beam of light is passed through the sample solution after passing through a
filter.
 The sample absorbs some light energy, transmitting the other.
 This transmitted light falls on the photocell that generates the photocurrent.
 This photo-current is recorded by the galvanometer or micro ammeter which is having
transmittance scale, since the photometer is directly proportional to the transmitted light,
the transmittance scale is linear.
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20. Single Beam Photometer - Steps in experiment
 With photocell darkened, the meter is adjusted to zero by zero adjustment.
 The blank or reference solution is inserted in the path of light beam and light intensity is
adjusted by means of rheostat in series with lamp.
 With this adjustment the meter reading is brought to 100 scale divisions.
 Solutions of both standard and unknown samples are inserted in place of blank and the
reading of the specimen relative to the blank is recorded.
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21. Double Beam Filter Photometer – Working Principle
 Double beam filter photometer consists of a source of light (tungsten filament lamp),
lens to make the light beam parallel, filter of wavelength selection, cuvette with sample
holder for keeping the solution to be analyzed, mirror to make incident the part of light
beam onto reference photocell, two photocells (one as reference and other as
measuring), potentiometers for zero and span adjustments and a recording device
(galvanometer).
 In double beam photometer, the light rays from the source are made parallel and passed
through a filter.
 It is divided into two parts; one part passes through the sample solution cuvette and falls
on the measuring photocell and the other part passes directly onto the reference
photocell.
 The galvanometer receives opposing currents from the two photocells.
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22. Double Beam Photometer – Steps in experiment
 With the lamp off, the galvanometer is adjusted to zero mechanically.
 The potentiometer R1 is adjusted for T=1 or A=0.
 Then with a lamp on blank solution is placed in the light path of measuring photocell
and potentiometer R2 is adjusted until the galvanometer reads zero.
 The solution to be analyzed is then placed in the light path of measuring photocell and
R1 is adjusted until the galvanometer reads zero, keeping R2 unchanged.
 The absorbance or transmittance can be read directly on the scale of potentiometer R1.
Since the potentiometer R1 is calibrated in terms of transmittance and absorbance.
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23. Double Beam Filter Photometer: Diagram
 In double beam filter photometer errors due to fluctuations of the lamp intensity is
minimized also the scale of potentiometer R1 can be made much larger in size than the
scale of a meter in single beam filter photometer.
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24. Spectrophotometer - Diagram
 Spectrophotometer is used for quantitative measurement of the reflection or transmission
properties of a material as a function of light.
 Single Beam (a) & Double Beam Spectrophotometer (b).
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25. Differences between Colorimeter & Spectrophotometer
Spectrophotometer Colorimeter
It measures transmittance or reflectance as a
function of wavelength.
It measures absorbance of specific colours.
It can be used for UV and IR as well as visible
colours.
It can only be used in the visible spectra.
Wavelength Range 200 – 1100 nm. (IR – UV) Wavelength range 400 – 1100 nm. (NIR –
Visible)
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26. Colorimeter Video Demonstration:
https://www.youtube.com/watch?v=yTabfxvMdCM
([Colorimeter] – Check Colorimeter Definition & Colorimeter Principle)
https://www.youtube.com/watch?v=vzShIdmvhxY
(video physics lumber brodum photometer tutir al abron)
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27. References:
 https://en.wikipedia.org/wiki/Colorimeter_(chemistry)
 https://www.slideshare.net/ApurvaJha1/colorimeter
 https://www.slideshare.net/PaulEagleYushau/colorimeter-76138764
 https://www.electrical4u.com/standard-photometer/
 https://myclassbook.org/single-beam-filter-photometer/
 https://myclassbook.org/double-beam-filter-photometer/
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