I would use Kubelka-Munk for the y-axis. Kubelka-Munk transform takes the diffuse reflectance data and makes it similar to absorbance - but it is not the same as absorbance.

If you want to take an absorbance spectrum of an insoluble powder, you have to make a very thin dispersion of it - mix the powder with a transparent material (a viscous oil is good - silicone oils are pretty transparent in the UV-Vis). Apply a thin film of the dispersion on a quartz slide. Use a thin film of the transparent material alone on a quartz slide as a reference. This will allow you to take a direct UV-Vis absorbance - it isn't ideal because of scattering by the particles, but it is the best you can do.

Ideally you can control the thickness of the film so they are identical - spin coating is the easiest way to make identical coatings, but you can also make a mechanical guide with thin wires that go across the slide - by running the edge of another slide across the wires the coating is the thickness of the wires - if you use very fine wires you can make a coating that is quite thin.

@ James E Hanson. Thank you so much.

Y-axis is K-M function, but in case of band gap determination, for direct band gap, it is (alpha.hv)2 and for indirect band gap, it is (alpha.hv)1/2.

Regarding measurement, I am not sure kind of instrument you have, but absorbance can be directly measured in absorbance mode using kbr or BaSO4 as standard. Basically you need do baseline correction with these standards and then mix your powder sample same standard to measure the absorbance in absorbance mode.

@Santosh Kumar. Thank you for your information. Sorry for the late reply. I have used UV-Vis-NIR spectrophotometer (Model: Cary 5000, maker: Agilent). In my case, the incident light can not pass through the powder sample. Do you think only change in absorbance mode is sufficient to to measure absorbance? The measurement procedure what you said, it will give the UV diffuse reflectance spectra of the sample and we need to convert it by Kubelka-Munk function, F(R). For liquid sample, as the light can pass through the sample, some part of the light will be absorbed and the detector can measure the absorbance. But in case of solid sample, light will fall on the sample and then it will reflect from the sample and the emitted light can be detected by detector. This emitted light is associated with the reflectance which we can measure directly.

Hi Atanu,

With UV diffuse reflectance kit, one can measure absorbance, K-M (F(R))and % reflectance of solid samples. Based on % reflectance, if am correct, the instrument calculate the absorbance or F(R). This is how we measure absorbance spectra in absorbance mode and F (R) or %R for band gap calculation in their respective mode given in the instruments.

Kind regards

Santosh

In continuous.... If I am correct the equations for F(R) and Absorbance (A) as follows:

F(R) = (1-R)2/2R

A = log(1/R)

Kind regards

Santosh

@Santosh Kumar . Thank you. I agree with you.