Spectrophotometer

From ProPHOTO WIKI

Introduction

Unlike the simpler colourimeter, spectrophotometers use a diffraction grating to break light down into its component wavelengths. A set of sensors then measures those spectral components directly and can provide an extremely accurate reading. As spectrophotometers are measuring colour by its underlying nature, these instruments will provide the highest levels of precision and are flexible enough to work on a wide variety of different output devices (monitors, desktop printers, printing presses, etc.). Unfortunately, this complexity also means that these instruments are typically much more expensive than other devices.

How it Works

Fig 1 - The general makeup of a spectrophotometer.

Fig 2 - A photograph of the sensor used in a simple spectrophotometer design.

Spectrophotometers use an optical mechanism to break down light entering the instrument into it's component parts and measuring them in an objective manner. To do this, light is first collected from the external target (see fig. 1.2) that the user is attempting to measure. This light is then passed through a set of optics (fig. 1.3) that focus and project it onto a diffraction grating (fig. 1.4).

This grating works like a prism and deflects light of different wavelengths at different angles. As such, the resulting patturn can be projected onto a set of sensors (fig 1.5 and fig 2) that can measure the light at specific intervals. As each sensor can measure the quantity of light in a specific area of the spectrum, this allows the instrument to relay the exact components of the light being measured to the computer.

This process is repeated for a set of differently coloured patches and the data from each measurement is stored. The profiling software can then compare the colours that the patches were supposed to be with the colours that were actually measured. This data is then used by the computer to generate a colour profile that can counteract the mistakes produced by the display device.

Reflective vs. Emmissive Targets

As noted above, spectrophotometers are instruments that measure light being projected into them. This works well for emmissive targets like monitors, as they produce their own light that can be measured by the instruments. When profiling devices like printers, however, the targets are reflective in nature and require an external light source to be measured.

In order to overcome this limitation, most spectrophotometers are equipped with an internal light source (fig 1.1) that can illuminate the target being measured. As the measured results are dependant on the quality of this light, these devices must produce even illumination and have known spectral characteristics. To ensure this is the case, many modern units have built in calibration mechanisms that periodically check this system.

Available Devices

• Gretagmacbeth Eye One Pro - Low cost handheld spectrophotometer aimed specifically at photographers.

See Also

• Articles on Colour - A collection of articles covering topics in this area.
• Colourimeter - A simpler calibration device that measures colour using a small set of filtered sensors.