Variable stars are generally analysed using photometry, spectrophotometry and spectroscopy. Gases in the cabin can be tested using gas chromatography, mass spectrometry and infra-red spectrophotometry. The methods used to identify these chemical compounds were chromatography, spectrophotometry, electrochemistry, and radiochemistry. Gas phase titration has several advantages over simple spectrophotometry. Using spectrophotometry for measuring the turbidity of cultures is known as turbidometry. Another way of measuring hematocrit levels is by optical methods such as spectrophotometry. These data are measured through spectrophotometry reports at certified testing laboratories worldwide. The amount of lignin is determined using gravimetry and spectrophotometry. The formation of indophenol is used to determine ammonia and paracetamol by spectrophotometry. Protein levels are evaluated through densitometry (how intense the stain is) or spectrophotometry. Optical flats have uses in spectrophotometry as well. Colorimetric equipment is similar to that used in spectrophotometry. The element is used as an internal standard in spectrophotometry. The most common and widely used way is through spectrophotometry. However, recent guidelines suggest that spectrophotometry should be performed.