Whiteness and Yellowness Indices Questions and Answers

Answer: A

Whiteness formulae compress a reflectance curve into one figure so that near white samples can be ranked in the order an observer would rank them. The number carries no chemical meaning and is not a simple reflectance reading at one wavelength.

Answer: B

Observers asked to select the whitest sample from a set consistently prefer one with a slight blue cast over an exactly neutral one of the same luminance. This preference is why every practical whiteness formula rewards a small negative b* rather than zero.

Answer: C

Preference studies show the opposite. Given equal reflectance, most observers choose the slightly bluish sample as the whiter of the two, which is precisely why blueing agents and fluorescent whiteners have been used for centuries.

Answer: D

A blueing agent is a very dilute blue or violet colourant. It removes a little of the yellow light that made the substrate look creamy, so the sample looks whiter. Because it subtracts light, blueing always lowers the luminance slightly.

Answer: A

A fluorescent whitener absorbs in the near ultraviolet, roughly 340 to 380 nm, and emits in the blue near 435 nm. The emitted light adds to the reflected light, so the apparent reflectance in the blue can exceed one hundred per cent.

Answer: B

Both are true and the reason gives the mechanism, and the effect is confined to the blue region where the whitener emits. Reflectance factor compares the sample against a perfect reflecting diffuser under the same illumination, and fluorescence supplies energy from outside the measured band.

Answer: C

Excitation occurs in the near ultraviolet around 340 to 370 nm. Emission is in the visible blue near 435 nm. The Stokes shift between the two bands is what allows invisible radiation to be converted into visible light and so raise the apparent whiteness.

Answer: D

Emission near 435 nm places the added light exactly where it compensates for the yellowness of most substrates. The complementary relationship between blue emission and yellow absorption is what produces the whitening effect.

Answer: A

A whitener needs ultraviolet excitation and adds blue emission, so the first two hold. The third describes blueing, which subtracts yellow light and therefore lowers rather than raises the luminance factor of the sample.

Answer: B

Fluorescent emission is proportional to the ultraviolet energy absorbed. Daylight is rich in ultraviolet whereas most incandescent and many fluorescent lamps carry very little, so the whitener contributes far less indoors.