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Lightness, Luminosity, Luminance and Other Tonal Descriptors
Images have two components: color and tone. Or is it luminance, luminosity or what? There is a wide range of terms used to describe this 'black and whiteness' quality. Here is a discussion and differentiation for each of these confusing terms. This includes consideration of photo editing variations, where converting a color image to black and white can have surprisingly different results. Please note that, just to confuse matters, there there seems to be limited agreement about definitions, so please take these as a result of research but not a definitive answer.
To help with understanding of computer color notes below, a quick summary of the effects of Red, Green and Blue (RGB) may be helpful.
Colors in computer processing are usually held as a combination of values of Red, Green and Blue for each pixel. Typically, in an 8-bit data scheme, each of these can range from 0 to 255. For example, Red=0 means 'no red in color' and Red=255 means (in 8-bit color) 'maximum, full-saturated, red'. For calculations, these values are often normalized to a range between 0 and 1 (calculated as Red/255, Green/255 and Blue/255). You can also use any number of bits for each RGB color, for example 12, 14, 16, 20, 24 or 32 bits. To normalize these to 0 to 1, again we simply divide by the maximum value they can have.
When Red, Green and Blue values are equal to one another, then the resultant color is gray. When they are all zero, the result is black, and when they are all 1 then the result is white. Hence, in an 8-bit color scheme, you can have values of 0 to 255, giving 256 black/white shades, which are normalized to the range from 0 to 1. In a 16-bit scheme, each RGB color can have 65536 shades, which can be normalized to the range 0 to 1 (by dividing by 65535). This gives many more color possibilities than 8-bit color, but results in much bigger files and requires much more computer power to process.
When used in reference to light (as opposed to sound), 'tone' is a general term for talking about the degree of light and dark within an area color. Hence when we talk about tone contrast, we are contrasting light and dark. 'Tone' can also be used in reference to a color cast, for example where white and grays have a tinge of another color (red, orange and yellow are called 'warm', while green, blue and purple are called 'cool').
'Tonal range' is a term that describes the difference between the darkest and lightest part of an image. A full tonal range will go from black through to white, though many images have a more limited range.
The human eye can detect a range of tone in the ratio of approximately 1 to 1,000,000. Photographic images tend to be far less with typical tonal ranges with ratio 1 to 200.
A 'tone scale' is a set of tones that are used to discuss and help control images, typically stepping between black and white in regular intervals.
In photo editing, a number of 'tone controls' are typically available, including Exposure, Brightness, Contrast, Shadows, Highlights, Mid-Tone, Levels and Curves.
Technically, luminosity of a light source is a measure of the light energy emitted, typically measured in Joules per Second. Imagine light as a cylinder of energy coming from a torch. Turn the torch on for a second and capture that cylinder (which will of course be quite long, given the speed of light). Or maybe have some transducer that turns light into heat (a simple black box around the torch will do), then turn on the torch for a second and measure how much hotter the box now is (so not that much energy).
The eye responds to light energy. A bright light is seen as bright because it has more energy, or 'luminosity'. Like any sensor, the eye can detect a range of incoming energy levels. Too low and nothing is detected. Too high and the energy damages the sensor. The brain knows this and will force us to close our eyes or turn away when the light is too bright.
In computer image editing, luminosity is one component of Hue, Saturation and Luminosity or HSL, which is an alternative to Red, Green Blue (RGB) representation of pixels. The L in HSL can also stand for Lightness or Luminance, depending on the source of the editing software used, which shows the looseness by which these terms can be used.
Technically, luminance of light is a measure of the luminous intensity per unit area of light travelling in a given direction. You can take the same amount of light energy and have it concentrated at a point or spread over a wider area. The point seems brighter than the area because the intensity per unit area is higher, even though the total energy is the same. In other words, luminance is a measure of light not over time (like luminosity) but over an area.
In the eye, more rods and cones are activated in seeing an illuminated area than a point of light (which is focused by the eye's lens on a smaller area within the retina). We are less dazzled by a lit area than a point of light because dazzling is a function of individual rods and cones, not many of them together.
In photo editing, luminance can appear as the L in HSL. It also appears in 'Luminance masking' where a layer mask is based on a limited range of dark or light within the image. This lets you edit these areas directly, for example making them more light or dark, or even changing their hue or saturation.
Lightness as a general term for the perception of brightness. It is the opposite of 'darkness', which it implies when there is talk of 'low lightness'. In conversation it often implies 'sufficient light' to see. It can be used as a generic term to talk about all kinds of light and dark, and is especially accessible for people who may be confused by 'luminosity' and 'luminance'.
In the eye, the lightness we perceive is not directly proportional to the arrival of photons. Rather, it is a cube root curve (which is close to an exponential curve). The effect is that when there is a little light, a little more is seen as a significant increase, while when there is bright light, the same increase is hardly noticed.
In photo editing, HSL can mean Hue, Saturation and Lightness, depending on the software you are using. Lightness is also often the same as 'Value'.
Physically, brightness is the perception of luminance. That is, a light source has luminance and so emits light, then the eye detects this and the brain translates the stimulation of rods and cones into a subjective perception of the light.
Brightness can refer to the total or average lightness of an image, where the brightness value of each pixel is summed and divided by the number of pixels. Cameras on automatic control typically try to set the average to a mid-gray. This is why unusually light photographs, such as of snow, may appear rather more gray than they should be.
In photo editing, Brightness is often paired with Contrast as a set of controls. Brightness is also found in HSB, which stands for Hue, Saturation and Brightness. In this, Brightness is the same as Value, in HSV adjustment. This in turn is a variation on HSL editing.
Value is another word for Lightness or Tone as a description of how we perceive how bright a light is.
In photo editing, value may be seen in HSV (as opposed to HSL) controls. A difference here is that while turning up L typically makes the image whiter, turning up V changes the colors, typically appearing more of a pastel appearance.
Luma is a measure of the brightness of a video frame (technically measured as the weighted sum of gamma-compressed RGB signals). It is often partnered with measures of 'chrominance', which is a measure of the 'colorfulness' or color saturation of the image.
Light representation of an image can be divided into the chromatic part, or hue and achromatic part, or tone, black/white, etc.
Chroma is a term that is particularly used in video for the color component of an image and achroma is the opposite of this, the non-color.
Saturation is the extent to which a hue is shown. When this is at the maximum, an image is called 'fully saturated'. When it is at the minimum, the image is called 'desaturated'. Making an image desaturated in a photo editor is typically done by finding the HSL (Hue, Saturation and Luminosity/Lightness) control and turning down the Saturation slider.
Desaturation moves a color towards gray. Saturation maximises the possible lightness of pixels while sustaining the R:G:B ratio, which
In photo editing, you can remove the color from an image by desaturating it, which is simply achieved by turning down the Saturation in the HSL control. Decreasing saturation is achieved by moving the RGB colors towards mid-gray. Red, Green and Blue are held as values which are normalized as a range between 0 and 1 (in an 8-bit system, this actually the range 0 to 255). In mid-gray, each color is half-way up this scale, in other words 0.5 (or 127 in the 8-bit system).
In photo editing, saturation is calculated as follows:
When an image is desaturated, appearing only in black and white, it is often described as 'grayscale'. This is because the image is visible as a series of shades along a spectrum from black to white, including all variants of gray (hence 'gray scale').
Early photographs were all grayscale and color only appeared later. Grayscale (and monochrome) is still a popular medium, especially in the more artistic fields. A reason for this is that it requires more skill to produce a good grayscale image as you no longer have color to create interest and meaning. Grayscale photographs often use strong tone contrast between dark and light to create impact.
In photo editing, there are often many options in converting a color image to grayscale, from just turning down the Saturation in HSL to making adjustments to the color balance before removing color hues.
Monochrome is similar to grayscale, in that is based on a spectrum between two colors, but these colors need not be black and white. Often, 'tints' in an image change the 'black' end of the spectrum to another color, such as sepia brown (which naturally appeared in early photographs). You can also change the light end of the spectrum to a light shade of such as yellow or red.
Monochrome images are the only option in printing when you have just black ink (as newspapers did for many years). When you cannot print shades of gray, dots of black are used in what is called 'half-tone'. In the same way that computer screens are made up of dots, if you view a half-tone image from far enough away, you see just grayscale.
When you see any of the above terms, understanding how they are defined (and perhaps defined rather loosely) can help you in deciding action from photo editing to conversational clarity. In photo editing, the use of light and dark is critical in making a 'good' or not so good image. Lighter areas attract attention, while dark areas repel the eye and can seem threatening. With such understanding, you can create images that are more influential, nudging people into feeling (and even believing) something different.
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