Which equation defines ITC?

• A. ITC (%) = Incident Intensity / Transmitted x 100
• B. ITC (%) = Transmitted Intensity / Incident Intensity x 100
• C. ITC (%) = 1 - IRC
• D. ITC (%) = (Transmitted + Incident) / 2

Answer: (B)

ITC measures the fraction of the incoming light that passes through the sample, expressed as a percentage. It is defined as the transmitted intensity divided by the incident intensity, times 100: ITC (%) = Transmitted Intensity / Incident Intensity × 100. This makes intuitive sense because if all the incident light gets through, the transmitted equals the incident and ITC is 100%; if none gets through, ITC is 0%.

The other forms don’t capture that same idea. Using the inverse ratio would describe how much incident light corresponds to a given transmitted amount, not the fraction that gets through. Subtracting from 1 with IRC only aligns with a different relationship (in some cases it resembles 1 minus reflectance, assuming no absorption), but it isn’t the general definition of ITC. Averaging the two intensities has no physical meaning as the transmission fraction. For these reasons, ITC is best defined by the transmitted-to-incident ratio expressed as a percentage.