How do I measure the optical density of objects using Openlab?

Technical Note: 49
Reads: 9921
Creation Date: 03/04/2000
Modification Date: 12/02/2004

The optical density of objects in a transmitted light image can be determined in Openlab by using the Custom Measurements feature in the Advanced Measurements module. The optical density (OD) of an object, for a given wavelength, is expressed as log 10 (1/T) where T is transmittance (ratio of transmitted power to incident power).

Open the Openlab document that you wish to measure and choose "Show Measurements" from the "Image" menu to open the Measurements window. In the Measurements window select the ROI table, and click on the record measurements button.

Draw an ROI in the image that defines the background . The ROI must not include any part of the object for which the OD is to be determined. The mean (pixel) intensity value recorded in the Measurements table is the value of the incident power of the illumination. Using a number of ROIs at different positions within the background will provide a statistically more accurate mean intensity value.

Now define the object(s) for which the OD is required either using one of the ROI selection tools or by using the Density Slicing module (How do I density slice an image?). The Measurements table will now include a series of mean intensity values for each object in the selected layer. Select "Custom Measurements" from the "Table" menu in the Measurements window (see How can I create my own columns in the Advanced Measurements table?).

If your image is >8 bit monochrome or 24 bit color, create a new column

OD = log10(p / mean)

where p is the mean intensity of the power of the incident illumination measured from the background ROIs.

If your image is 8 bit monochrome then the equation for the OD must be re-written because the Measurements table records index values not intensities, that is OD = log10((255 - p) / (255 - mean))