Lightroom’s Tint Slider and the Planckian Locus

In Lightroom’s develop module, the top two sliders are the colour temperature slider (“temp”) and the tint slider. Most of us are familiar with colour temperature: the higher the number along the Kelvin scale, the cooler or bluer the light looks. The scale is back to front in Lightroom so that the higher numbers along the axis represent yellow and lower numbers blue. Perhaps this is because we are often correcting colour temperature rather than observing it.

The second slider is the tint slider. This follows a green to magenta axis, which is less intuitive than blue to yellow. We know that daylight varies in colour. It ranges from a warm, yellow hue in early morning or evening sunlight to a bluer colour during the middle of the day. Natural light follows a path akin to that of the Lightroom temp slider. So, what is the tint slider for?

Look at the diagram below. It shows the Planckian locus, which defines the path of colour temperature. The locus is orange at 1500K and gradually turns yellow then blue. Light sources created by heat follow this path or, in the case of daylight, one next to it (the daylight locus). The lines you see crossing the locus represent “correlated colour temperatures”. Note their green to magenta axis at the 6000K daylight mark.

Planckian locus, correlated colour temperature, fluorescent lighting, LED lighting, Kelvin scale, colour temperature, color temperature, tint slider
The Planckian locus represents the Kelvin scale, whereas the lines crossing it are “correlated colour temperatures”. (Image: Wikipedia)

Fluorescent and LED light sources never have a “true” colour temperature. Instead, they have a correlated colour temperature. They are prone to marked variation in hue along a green to magenta axis. If you’ve ever seen a green or magenta colour cast when trying to calibrate a monitor, this is because of fluorescent or LED backlighting. Filament lighting is more consistent in hue and always sits on a precise point along the Planckian locus.

daylight locus, natural light, colour temperature, color temperature, tint slider
The daylight locus plots the chromacities (colour qualities) of daylight through its different phases. It’s represented here by the upper curve, with the Planckian locus below it. In LED or fluorescent lighting, chromacities such as D50 and D65 are inherently more precise than correlated colour temperatures. Image by Adoniscik GFDL or CC BY-SA 4.0-3.0-2.5-2.0-1.0]

Now we can see a potential use for the Lightroom tint slider: artificial lighting. There are other situations where it might be handy. Light reflected off grass creates a green tint in nearby objects, for instance.

The “White Balance Selector” in Lightroom evens up RGB values, thereby removing any colour cast. It affects all the RGB colour channels discussed in the previous blog entry. Thus, it alters both the temp and tint sliders (blue to yellow, magenta to green).

Pulling Histograms Apart

There are various types of histogram in photography, including RGB, luminosity and “colors”. Lightroom uses the latter, since it shows which RGB colours are being clipped and to what extent. “Clipping” is underexposure or overexposure. It shows as data banking to the left or right of the histogram. If all three red, green and blue channels clip in unison, the tone is either pure black or white and no detail is retrievable.

In Photoshop CC and on many camera LCDs, you have the choice of viewing discrete RGB histograms. This makes it easy to see colours being clipped*, since the data is not packed into one small graph. It’s also useful for assessing colour balance, because peaks in the three histograms align when the colour is neutral. Photos with a strong colour cast or bias yield uneven histograms.

Primary and Secondary Colours

To help you understand separate RGB histograms, it’s handy to know that they each represent a range of colours. For instance, a fully saturated red will cause data to bank over to the right of the red histogram. If the data was to bank to the left it’d indicate the opposite: pure cyan. Cyan is the secondary colour that opposes red on an RGB colour wheel. Thus, you can think of these histograms as cyan to red, magenta to green and yellow to blue.

RGB histograms, red histogram, green histogram, blue histogram, color, colour balance, colour editing
Though not shown this way, individual RGB histograms represent the three primary colours and their secondary opposites.

When making levels adjustments in Photoshop using the separate RGB channels, moving the left or middle slider to the right increases the secondary colour. Equally, moving the right or middle slider left strengthens the primary colour. You wouldn’t usually make these edits unless getting rid of a colour cast. Similarly, in a curves adjustment, pulling the red, green, or blue curve down boosts the secondary colour.

*If shooting raw files, the camera’s histogram does not depict exposure latitude as precisely, since it’s derived from a JPEG.