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 the concept of colour temperature: the higher the number along the Kelvin scale, the cooler or bluer the light appears. 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, ranging 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. What, then, is the tint slider for?

Study the illustration below. It shows the Planckian locus (aka the blackbody locus), which defines the path of colour temperature. The locus is orange at 1500K and gradually turns yellow and finally 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 Planckian locus represent “correlated colour temperatures”. Note their green to magenta axis at the 6000K daylight mark.

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 significant 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. Incandescent lighting is more consistent in hue, always sitting on a precise point along the Planckian locus.

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 more natural situations where it might be handy. Light reflected off grass creates a green tint in nearby objects, for instance.

Note that the “White Balance Selector” in Lightroom levels RGB values, thereby removing any colour cast when used correctly. It affects all the RGB colour channels discussed in the previous blog entry. Thus, it will alter both the temp and tint sliders (blue-yellow, magenta-green).

Pulling Histograms Apart

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

In Photoshop CC and on many camera LCDs, you have the option of viewing individual RGB histograms. This makes it very easy to see colours being clipped*, since the data is not crammed into a confined space. It’s also useful for evaluating colour balance because peaks in the three histograms will always align if the tone is neutral. For instance, a white cloud will create three similar peaks to the right of individual RGB histograms. Photos with a strong colour cast or bias produce uneven histograms.

To help you better grasp 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: saturated cyan – the secondary colour opposite red on an RGB colour wheel. Thus, the three RGB histograms are effectively cyan-red, magenta-green and yellow-blue.

RGB histograms
Although never displayed this way, individual RGB histograms represent the three primary colours and their secondary opposites.

When making levels adjustments in Photoshop using the individual RGB channels, pulling the left-hand or middle slider to the right increases the secondary colour, whereas moving the right-hand or middle slider left makes the primary colour stronger. You wouldn’t ordinarily want to make these adjustments unless removing a colour cast. Similarly, in a curves adjustment, pulling the red, green, or blue curve downwards 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.