You can use a spectrophotometer to measure the transmittance and calculate the absorbance. Also you can use this tool.

http://www.epolin.com/converting-absorbance-transmittance 

Thanks a lot. But how do I make a slide fit in the spectrophotometer?

Dear colleague A. S. Vishwanathan,

Thin films were prepared by the thermal evaporation of bulk material onto cleaned glass as a substrate (link of types of substrates is attached)  using an Edward coating unit model E306A under a vacuum of 10^6 Torr (link attached). the spectrometer is designed to use the mentioned substrate or you can cut to wanted size. You can adjust  the parameters that you want to measure on the configuration of the spectrometer such as transmittance. T, reflectance, R or absorbance, A. through the wanted range; UV visible or  IR region. if you measured  .T and R you can you use the Equation T+R+A=1

A= 1-(T+R) or you can measure A directly form the spectrometers. i wish that help you.

the attached two papers may be useful.

yours

A. Eldenglawey

https://www.google.com.sa/search?hl=en&authuser=0&site=imghp&tbm=isch&source=hp&biw=1280&bih=699&q=microscope+slides&oq=microscope+s&gs_l=img.1.0.0l10.1634.11262.0.15530.12.12.0.0.0.0.555.3361.0j2j7j1j0j2.12.0....0...1.1.64.img..0.12.3356.NX7ycu38QPA

https://www.google.com.sa/search?hl=en&authuser=0&site=imghp&tbm=isch&source=hp&biw=1280&bih=699&q=edwrds+coating+unit+306A&oq=edwrds+coating+unit+306A&gs_l=img.3...3493.26124.0.27168.24.24.0.0.0.0.479.5662.0j6j15j0j2.23.0....0...1.1.64.img..1.10.2517.KpsjQnr4CTo

Another way it to use a Variable Angle Spectroscopy Ellipsometry (UV-VIS). Just as described by Pravin for the spectrophotometer, perform baseline correction using your substrate and then continue to do your transmission data scan scan using baseline data. This gives you transmission data which can be used to estimate A, using ;

A=1-(T+R)

For most transparent thin films R can be assumed to be zero, otherwise it may need to be determined. 

Thank you very much.