Main Difference – Colorimeter vs. Spectrophotometer
Colorimeters and spectrophotometers are both used to measure colour-absorbing properties of substances. In chemistry, they are especially used to measure colour absorption by solutions. The main difference between colorimeter and spectrophotometer is that colorimeter is a device which measures absorbance of specific colours, whereas a spectrometer measures transmittance or reflectance as a function of wavelength.
What are Colorimeters
Colorimeter is a device which measures how much of a specific colour of light is absorbed by a solution. A colorimeter comes either with a set of coloured filters or with LED bulbs that emit specific colours of light. To use a colorimeter, first, the appropriate colour has to be selected. Then, a cuvette containing the solution is placed inside the colorimeter. The colorimeter will then give the absorbance for the particular colour selected. It is important to remember that a solution of a given colour actually absorbs its own colour the least. For instance, a green solution containing chlorophyll will absorb green colour the least.
A colorimeter in use
According to Beer’s Law, the absorbance of a colour is directly proportional to the concentration of the solution (provided solutions of the same chemical is measured and that the path length of light remains unchanged). If a graph of absorbance vs. concentration for solutions of known concentrations is drawn, then the graph can be used to measure concentrations of unknown solutions.
What are Spectrophotometers
Spectrophotometers measure transmittance and reflectance of light as a function of the wavelength of light. That is, it measures transmittance and reflectance for all colours of light, and shows how the transmittance/reflectance varies as the colour of the light is changed. Unlike with a colorimeter, the range of wavelengths that can be measured with a spectrophotometer extend beyond the visible range into infrared and ultraviolet regions of the electromagnetic spectrum. The figure below shows a spectrophotometer:
The graph below shows the absorbance as a function of wavelength for chlorophyll (transmittance measured from the spectrophotometer can be converted to an absorbance value). You can see that green light is absorbed the least, hence chlorophyll looks green. Blue and red are the colours absorbed most by chlorophyll. (Sometimes in laboratories, plants are grown under red or blue lights to ensure that the leaves absorb light most efficiently) :
Chlorophyll Absorption Spectra
Difference Between Colorimeter and Spectrophotometer
Function
Colorimeter measures absorbance of specific colours by a sample.
Spectrophotometer measures transmittance or reflectance of colours in a sample, as a function of wavelength.
Range
Colorimeter works with only light in the visible part of the electromagnetic spectrum.
Spectrophotometer works with infrared and ultraviolet light as well as visible light.
Cost
Colorimeter is cheaper compared to spectrophotometers.
Spectrophotometer has a much wider range of functions, which includes functions of a colorimter. Hence, it is more expensive than a colorimeter.
Image courtesy:
“A water sample from Gaziantep, Turkey, is inserted into a colorimeter May 20, 2013…” by U.S. Senior Airman Chase Hedrick/Air Force photo (This file was derived from: Chlorophyll ab spectra2.PNG) [Public domain], via Wikimedia Commons
“Unicam 5625 UV/Vis Spectrophotometer” by Skorpion87 (Own work) [Public domain], via Wikimedia Commons
“Chlorophyll ab spectra” by Aushulz & M0tty (This file was derived from: Chlorophyll ab spectra2.PNG) [CC BY-SA 3.0], via Wikimedia Commons
