Photometer vs. Spectrophotometer

Photometer

A photometer isolates a specific wavelength of light by using filters. A colorimeter uses edge band filters, or some similar system, to separate light out into colour components, and then fits those to matching curves based on the human eye, to produce colour values based on what the human eye would see. This is ideal for matching the human visual response, but tells you nothing about data invisible to the human eye, such as emissive spikes at narrow points in the spectrum; that’s spectral data, and requires a spectrophotometer.

Spectrophotometer

Spectrophotometers are different from photometers as they allow for measurement in the spectrum of all wavelengths of visible light and not just pre-specified wavelengths. Spectrophotometers work by isolating light at specific wavelengths from white light. A spectrophotometer breaks light up into a spectrum, using a colour grating or similar system. Then an array of sensors reads each section of the spectrum, producing spectral data. This is ideal if you are analysing the spectral emissions of a light bulb, a star, or some other light source, which is why spectrophotometers are often used as scientific devices.

Optical Configurations of a Spectrophotometer

Single Beam

In a conventional single-beam spectrophotometer, the blank and the sample are measured consecutively, with an interval of several seconds for a single wavelength measurement and up to several minutes for a full spectrum measurement with a conventional instrument. Lamp drift can result in significant errors over long time intervals.

Double Beam

The double, or dual-beam, spectrophotometer was developed to compensate for these changes in lamp intensity between measurements on blank and sample cuvettes. In this configuration, the light source emits a single light beam which is split by a light chopper, creating two beams of equal energy with equal optical path. One beam passes through the reference while the other beam passes through the sample.

Compared with single-beam designs, dual-beam instruments contain more optical components, which reduces throughput and sensitivity. For high sensitivity, long measurement times may be required. In addition, the more complex mechanical design of the dual-beam spectrophotometer may result in poorer reliability.

Split Beam

The split-beam spectrophotometer resembles the dual-beam spectrophotometer but uses a beam splitter instead of a chopper to send light long the blank and sample paths simultaneously to two separate but identical detectors. This configuration enables the blank and the sample to be measured at the same time. The split-beam design is mechanically simpler than the true dual-beam instrument and requires fewer optical elements.

 Best Uses for a Spectrophotometer 

Today’s spectrophotometers are designed to be both durable and portable, offering flexibility in use. While the applications are nearly endless, some of the best uses include:

• Elemental determination for water quality
• Enzymatic analysis in wine
• Analysis of fertilizer properties for agriculture