A "negative absorbance" means your reference is absorbing more than your sample.

A "negative absorbance" means your reference is absorbing more than your sample.

You have a "zero error". You can correct this by displacing your zero reference.

@Teodoro S Kaufman, we may not conform to zero error? How we can conclude to zero error?

Besides thechnical instrumental effects, it is necessary to worry about change in reflection and interference effects, see also the discussion

https://www.researchgate.net/post/How_can_I_deal_with_the_bleaching_effect_in_FTIR_spectroscopy

If I understand you correctly, the radiance measured after the absorption cell is higher than the input one, right? This can happen if the detector's sensitivity is higher at longer wavelenghts and there is a luminiscence at these wavelengths.

Your blank sample must show zero absorbance . Them you can get rid of -ve absorbance

when your glass samples act as lens you get apparent negative adsorbance.

Are you becoming "negative" absorption in the whole spectrum and can you change the scan rate (instead of uv->vis to scan vis->UV)?

Are you measuring transmittance and reflectance and then calculating the absorption? Is your coating used for antireflex coatings?

Are you calibrating with just one reference or you can test with several references?

We have the same problem - negative absorbance in the short-wavelength UV region for organic scintillation polycrystals. I think  Artem G. Feofilov gives suitable explanation.

Maybe the wavelength value use to measure your sample is not suitable.

In prinicipal a negative absorbance means that you have a gain medium. For reasons of energy conservation, however, there must be a preceding (pump) process. Typical examples are laser media.

I am getting negative reflection in UV Vis Nir what does it mean? And how to calculate refractive index from it? 

@Madhu Verma: To my best knowledge a negative reflection in the sense that R < 0 is not possible. Even if your sample would send out more photons than are incident, this would just mean that R>1. How did you measure and calculate the reflectance and which kind of sample did you measure?

P.S.: There is a phenomenon in literature termed "negative reflection" which describes the fact that both reflected as well as the incident light waves are at the same side of surface normal (non-normal incidence assumed), but I assume you don't refer to this phenomenon.

@ Dr Thomas: I also got negative reflectance for some samples. I used Si wafers for UV visible measurements. As Si is opaque of light so the incident and reflected wave will be on the same side of the surface normal. But how could I explain it? I mean how could I represent it according to the light absorption?  Negative reflectance means absorption is greater than 100% how is it possible? 

In case of Si you won't find that the incident and the reflected light is on the same side of the normal, at least to my best knowledge. What have your really measured in what way? Please give me more details!

Press autozero before proceeding the sample for analysis

{https://www.nature.com/articles/122012b0.pdf} The report by Raman and Krishnan 1928, Negative absorption of radiation, 1928 shows evidence of such phenomena.

this the third type of absorb/emits described by Einstein. This verified by Raman and Krishnan (2 eminent physicists of the time).

“Careful measurements have shown that the difference between the incident and scattered frequencies is exactly equal to an infra-red frequency of the molecule, so that the process of modified scattering involves the absorption of radiation by the molecule.”

This experiment used Benzene liquid, mercury light filtered to give only 4358A group light, then measured the emission in IR...

“the exciting 4358·3 line, and the measurements show that their frequencies (the emission frequencies ) exceed that of the latter by the infra-red frequencies of the molecule, namely, those corresponding to 16·55micron and 10·10micron respectively. The presence of these lines proves simultaneously the existence in the liquid of molecules at levels of energy correspondingly higher than the normal, and the fact that the incident radiation induces a return to a lower state of energy ; in other words, that there is a negative absorption of the radiation.”? The incident light 4358 nm produce IR light measurement at 10.10μ and 16.55μ which represent the wavelength of IR absorptions, thus, the benzene when exposed to light it can have –negative absorb and emits lights at higher wavelength or at lower energy level thus it drops to lower energy and emits light = incident +(-negative releases of existing excited state) thus emited at different wavelength.

Mohsen Mabrouk How large is the relative amount of photons that undergo inelastic scattering relative to those which are elastically scattered? Do you really think this amount can be made responsible for negative absorbance? Furthermore, strictly speaking the use of absorbance actually excludes scattering generally...