Table 1.

Glossary of terms for different aspects of the visual signals of flowers

Visual signal	Perceptual counterpart	Measurement	Optical characteristics	Current evidence for functional significance for visual signalling in nature
Dominant wavelength	Hue	Reflectance spectrum measured with bifurcated probe or integrating sphere and a colour space model.	Wavelength-selective absorption by pigments.	Important; many pollinators have innate and learned preferences for specific hues. Effects of dominant wavelength cannot always be disentangled from colour contrast.
Spectral purity	Saturation	Reflectance spectrum measured with bifurcated probe or integrating sphere and a colour space model.	Colourfulness of the stimulus as opposed to greyness; characterized by the slope and amplitude at the inflection points of the reflectance curve.	Has been shown to be important for a few bee species. Effects of spectral purity cannot always be disentangled from colour contrast and dominant wavelength.
Intensity	Brightness/luminance	Integrating sphere or other technique that allows measuring the absolute amount of reflectance.	Amplitude of the reflectance curve.	Not known to be important for diurnal pollinators.
Colour contrast	Perceptual contrast between two colours as detected by all photoreceptors. Depending on the species, this may be affected by dominant wavelength and/or spectral purity.	Reflectance spectrum and colour vision model.	Difference in the amplitude of the stimuli’s reflectance spectra in the visible range of wavelengths.	Important – many species use colour contrast to distinguish and/or detect flowers from close and large distances. Increased colour contrast has been shown in several bee species to increase the probability of correct target flower identification. The relative importance of hue versus saturation can be difficult to discern.
Green contrast	Perceptual contrast between two colours as detected by the green photoreceptor.	Reflectance spectrum measured with bifurcated probe or integrating sphere and excitation values of green photoreceptor for background versus stimulus.	Difference in the amplitude of the stimulus’ and background’s spectra in the green wavelength range.	In honey bees important for long-distance detection. In many insects vital for motion processing, and the processing of complex patterns, shapes or sizes of objects such as flowers.
Specular reflection	Gloss	Angle-dependent reflectance measurements	Specular (mirror-like) reflection. Gloss requires very smooth and flat flower surfaces (Fig. 4).	Unclear. No evidence supporting or rejecting gloss as a signalling function.
Iridescence	Angle-dependent coloration	Angle-dependent reflectance measurements	Physical interactions of light waves with periodically ordered nanostructures that differ in refractive index.	Very little or no functional significance in natural conditions.
Polarization	Gloss	Angle-dependent reflectance measurements or high-quality photo camera with polarization-sensitive filter.	Geometrical orientation of the oscillation of light waves. Polarization effects generally occur at smooth and flat surfaces and often co-occur with gloss (Fig. 4).	Very little or no functional significance in natural conditions.
Fluorescence	Fluorescence	Fluorescence microscopy.	Emission of light by pigments.	Very little or no functional significance in natural conditions.

For each aspect, we list the perceptual and physical counterpart, how it is measured, its optical characteristics and a conclusion as to its currently believed importance for visual signalling to pollinators in natural conditions. For details and calculations see Supplementary Data File S1.