Plug the spectrophotometer in and turn it on. Wait 30 minutes for the spectrophotometer to warm up to its operating temperature.
Rotate the zero adjustment knob to show a transmittance of zero while the spectrophotometer's cuvette chamber is empty. A cuvette is a special type of container that holds the solution you are testing. Line the needle up with its reflection when you take a reading from the spectrophotometer. This prevents parallax error when taking the reading.
Turn the wavelength selection knob to select the wavelength of light for which you wish to measure the transmittance.
Fill the blank cuvette with the test solvent. This solvent is frequently distilled water, although some spectrophotometers use a specific solvent. Clean the cuvette with the polishing cloth for your spectrophotometer and place the cuvette into your spectrophotometer's cuvette chamber.
Rotate the blank adjustment knob until the spectrophotometer's absorbance reading is zero. Remove the blank cuvette from the spectrophotometer's cuvette chamber.
Pour the test sample into the spectrophotometer's specimen cuvette. Clean the specimen cuvette and insert it into the cuvette chamber. Take the reading of the test sample to obtain the sample's transmittance for the wavelength of interest.
Repeat steps 2, 4, 5 and 6 to test additional samples with your selected wavelength.