Pictures; it’s all about light

In photography there are a number of properties of light we must understand:

  • Tone
  • Contrast
  • Intensity
  • Brightness
  • Colour
  • hue
  • Saturation

This article will endeavour to explain the differences of tone, intensity, colour, brightness, contrast, hue and saturation through a look at how these properties come about from the technologies and materials that produce pictures.

When we produce a picture it is either through printed media, transparency media, or electronic media. Printed media deals with modifying reflected light, electronic media deals with generating light and transparency media deals with a bit of both. Light is a mysterious energy that seems to comprise of electromagnetic wave energy as well as particle energy. Increasing the wave energy amplitude increases the light intensity…it looks brighter, while altering the frequency of the wave by varying the wave length alters the hue…that is our eyes are sensitive to the wavelength and we see this variance as colour shift.

There are a number of ways to alter these properties of light. We can absorb it using different materials, bring more than one source together, or regulate the source outputs. When light is reflected off a surface, some of it is absorbed by the surface matter. Different matter affects different wavelengths causing us to see these variances from the reflected surface as colour, tone and intensity shifts. So with print media, we can apply different compounds to the surface to give off different colours when light is reflected from it. These compounds can also determine how much light is reflected, giving us a sense of intensity. So all we have to do develop these compounds and place them on a medium designated to hold our picture. This is the principle of print media.

In print media, tone is represented by the amount of reflection, or its’ reciprocal absorption, across the full wavelength spectrum. A compound that absorbs all wavelengths evenly more than another compound will represent a darker area, while the reverse represents lighter areas. This darker and lighter property is referred to as tone.

Tone can range from black (no light is reflected, it is all absorbed), to white, where most light is reflected and very little is absorbed, to an infinite range of greys in between. Of course the human eye can not resolve infinitely small variances! The tonal variances between black and white determine contrast. Increasing contrast will eliminate some of the variances between adjacent areas, so a clearly defined jump in tone is detected, whereas decreasing contrast will introduce more tonal steps between adjacent areas giving a more gradual tonal change, or a flatter look on the tonal scale. So contrast is a measure of tonal range.

Intensity or brightness also interacts with tone, but it shifts all tonal steps proportionally up or down. With printed media, brightness is determined by the absorption properties of the medium, the mix of the compounds and the structure of both on a molecular level.

Some compounds we find easy to work with in these applications include dyes, inks, paints, pigments and silver compounds, not to forget the surface treatments that influence absorption/reflection.

When it comes to transparencies such as slide projections, these compounds still absorb light, only now it is filtered as it is passed through, rather than reflected as with print media. Intensity can be controlled through controlling it at the light source.

Electronic media represents a totally different concept. Light is not reflected at all but rather generated. At a certain wavelength we see light as white. Adjusting the intensity changes how bright this white light is. With no light we see blackness. As the wave amplitude in the visible light spectrum increases we start to see the blackness turn grey, all the way up to pure white. This amplitude is governed by either the amount of energy we supply to the light generation source, or the amount of filtering between the source and the eye.

LCD (Liquid Crystal Display) technology relies on light being reflected off a background through a glass display, which has an embedded grid of cells containing transparent liquid crystal material that can change its’ opacity with the application of an electrical charge. Thus it filters out more light in one state than the other and by being selective on the grid we can create patterns that make up our picture. Contrast is controlled by applying a different intensity or opacity control to individual cells on the grid, while brightness adjusts this control signal evenly, or proportionally over the entire grid. In some cases brightness might be achieved by adjusting the background light intensity. CRTs (Cathode Ray Tubes) as in the old TV’s and monitors of last decade and modern Plasma and LED (Light Emitting Diode) screens all work differently, however the principles for all electronic media are all similar.

Adjusting the intensity of individual cells, or light sources gives us tonal contrast control, while adjusting the intensity uniformly across the whole grid gives us brightness control.

So far all of this governs the amplitude of the light wave. To see colour, we need to vary the wavelength or change the frequency of the light waves at certain points on the grid. This change of wavelength results in a hue. The difference in hue and colour is slight to the eye. Hue relates to visible light wavelength, whereas colour also includes tints and shades that result from tonal variations of a particular hue.

One way to vary wavelength is to reduce the amplitude of the unwanted hues using filters leaving the desired one at full amplitude before applying contrast and brightness level controls. Another way is to mix different hues from different sources at a proximity to close for our eyes to resolve, so our eyes pick up a new hue because they can’t resolve the individual sources. If a pixel (one cell in the grid) contains three light sources, one emitting the red hue, another a green hue and the other a blue hue, all at the same intensity, then we see a modulated wavelength that represents white light to the human eye. If the intensity of any one of the tree sources changes, so to does the wavelength modulation and the overall hue. So to change the hue of a single pixel, all we need do is adjust the intensity control to each RGB element of a pixel individually. To change the tint or shade, we adjust the intensity of the tone to each pixel of a given hue, that is all RGB elements making up a pixel are adjusted equally.

By now you should be starting to see that the control signals going to each of the three RGB elements that make up a single pixel are complex and derived from algorithms that take in data from hue, contrast, brightness, saturation and vibrance controls, to mention a few. Some signals apply to individual RBG elements across many pixels, while others adjust hole pixels equally (contrast), yet others adjust all pixels on the screen equally (brightness), with all these control signals coming in on top of each other.

Recapping here, with print media, hue, contrast and brightness are achieved through the combination and mix of chemical compounds that absorb different parts of the visible light spectrum by different amounts. It is chemistry that governs the properties of reflected light and for every electronic algorithm, there is a chemical compound mix…well in theory anyway. It just requires good standards to ensure the same electronic data results in the same chemical compound mix during printing.

Now you might ask…what is saturation and vibrance?
Saturation is removing the greys from a hue.

Now you might be prompted to ask the difference between saturation and vibrance! Check out this site for an answer: Analyzing-photoshop-vibrance-and-saturation

If you learnt about colour through painting art studies, you learnt that mixing blue and yellow for instance gives you green. Changing the ratios of blue to yellow gave you a range of green hues. Adding back or white also changed the hue, but not in a way that could be replicated purely by blue to yellow ratio combinations. So although the hue might change with the addition of black or white to our eyes, it is easier to understand that the hue only changes with the combination of blue and yellow mix and it is the tint or shade that changes with the addition of black or white…same hue, different tone!

Monochrome verses monotone

Monochrome is another word for one colour, or one hue with varying underlying tones. It also includes no hues, to produce black, grey and white images, as both black and white are considered as a part of colour, even though technically not a hue.

Monotone is one tone. All black, or all white, or all one shade of grey. Not much of an image I would say. However you can have many hues present in different shapes and patterns over the monotone, to produce an abstract painted effect.

The histogram shows the percentage of each shade of grey starting at black on the left and running through to white on the right. The percentage is represented by the height on the vertical axis, while the tonal range is represented by the width on the horizontal axis. This tonal range can also be seen for each of the three primary hues; Red, Green and Blue (RGB), through options in the histogram tool.

There are a number of subtleties in the properties of light. For instance luminance is considered a measurement of direct light, not reflected light, however both direct light and reflected light have a measure of intensity. Lucky this remains in the realm of scientists, mathematicians, engineers and programmers who create the algorithms that light meter programs are built upon.

Adjusting the intensity upward in post-production should make the whole histogram move further toward the white end than the darker end. However pure black should remain black and not move at all on a histogram as there is no data recorded for pure black and you can’t change what you don’t have!. In reality, not all black is pure black and adjusting the intensity upward far enough will result in blacks moving toward greys. So moving the whole histogram relates to brightness or intensity adjustment, while stretching it out over the entire length increases tonal range.

Contrast is increased by reducing some of the greys across the tonal range. This would be represented by a histogram with a higher amplitude at both the black and white ends of the scale. Remember the amplitude is a measure of the percentage of a particular tone present in an image. Digitally tones are presented in fixed increments. The electronics might create 256 shades of grey including black and white. If certain grey shades are attenuated they will be seen as dips between spikes along the histogram. Basically the smoother the histogram the flatter the contrast.

Researched and written by Gavin Lardner CC BY 2011


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