Bio-Rad Image Lab™ Software - Manual (Page 14)

1 | Introduction
14 | GS-900 Calibrated Densitometer with Image Lab Software
Measurement of Optical Density
The GS-900 calibrated densitometer uses internal calibration standards to ensure
accurate optical density measurements of scanned samples. The internal calibration
standards consist of either a piece of photographic film (for transmissive scans) or a
piece of photographic paper (for reflective scans) that have been exposed in order
to generate several areas, or steps, with different optical densities. The optical
densities of these steps are accurately measured before being installed into the
instrument, and the instrument is preprogrammed with these values.
Prior to each scan of a sample, the appropriate calibration standard is scanned and
the light intensity levels for each step of the standard are used to correlate the
sample’s detected light intensity to a known optical density found on the calibration
standard. This allows light intensity levels measured on the sample area to be
correlated to a known optical density value and ensures accurate and reproducible
optical density measurements of scanned samples.
During manufacture, the internal calibration standard is measured using an
instrument capable of diffuse density measurements. However, the internal
calibration standard diffuses light differently than protein bands on a Coomassie
stained gel. Additionally, the light collection angles of the optics in the GS-900 make
it unsuitable for diffuse density measurements. For these reasons, specular density
measurements are more appropriate for Coomassie stained gels that are imaged
with the GS-900, and Image Lab reports specular density values only.