Optical density - a measure of the attenuation of light transparent objects (such as crystal, glass, photographic film) or reflection light opaque objects (such as a photograph, metals, etc...).

Calculated as the decimal logarithm of the ratio of the radiation flux incident on the object to the stream of radiation passing through it (reflected from it), that is, it is the logarithm of the inverse to the transmittance (reflectance). See also

http://physics.tutorvista.com/fluid-dynamics/optical-density.html

Best regards.

Is it a coating on a substrate? If you are using a blank non-scattering substrate as a reference, then your "absorbance" corresponds to the sum of reflectance and true absorbance. To reliably destinguish between these two (I mean quantitatively) is tricky, especially if you measure only specular transmittance. My short answer: forget it. If you need reflectance, search for a sufficiently equipped lab to perform appropriate reflectance measurements.

Thanks Mr. Mirgorod and Mr. Kubrin

My sample are semi transparent thin films, substrate is not used.

Self-supporting thin film? How thin? I am just curious because the answer stays the same - you better properly measure reflectance. I could imagine that it is somehow possible to derive reflectance by measuring absorbance on several samples of different (and precisely known) thickness by Kubelka-Munk approach but it will be everything else but straightforward. You need a (diffuse) reflectance setup.