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Old 15-01-18, 08:42 PM
Koo Schadler's Avatar
Koo Schadler Koo Schadler is offline
Tempera Painter
Join Date: Dec 2003
Location: Alstead, NH & Zirahuen, Mexico
Posts: 318
Default Luminosity

Hello Rebecca,

Relative to your last post, in which you asked about luminosity, pasted below is a handout on luminosity. As always I welcome anyone's thoughts, comments, corrections.


Egg tempera is often described as a “luminous” medium. What does this mean, and how is it achieved? Luminosity means different things in various disciplines and may be explained in technically dense language. My hope is to explain in simple terms what I believe makes paintings appear luminous, and why egg tempera is especially good at achieving this.

What is luminosity?
I asked a dozen painters this question and no two answered alike. However all mentioned the primacy of light - a luminous painting appears to emanate light. There are different ways to create radiance. Depending on the medium, working method and goals of a painter, he or she can employ some or all of these methods.

To see an object there must be light. The first step in creating luminosity is to understand the light source illuminating your subject matter, and consistently render its effects throughout a painting.

We live in a world illuminated by a one-source light, the sun. Single source, full-spectrum lighting creates consistent characteristics on objects it shines upon. Replicate some or all of those characteristics, and you create an impression of light, or luminosity. These characteristics include:

- Transition from light to dark.
Illumination creates value transitions from light to dark on subject matter. The greater the value difference between the light and shadow areas, the more dramatic the lighting appears.

-Vary chroma
Illumination also affects a subject’s local color; it varies in purity, ranging from bleached out in the highlight, to varying degrees of saturation in the light, halftone and shadow areas.

-Transition from opaque to transparent paint
Highlights (the areas of greatest illumination) have solidity, which is best conveyed by dense, opaque paint (impasto, if possible). Shadows are atmospheric, rendered with transparent paint.

- Cast shadows and reflected lights
A single light source creates cast shadows and, in some instances, reflected lights. Rendering these characteristics is another way to denote that an object is being illuminated.

When a light source is large and close to a subject, it creates “soft light” – light that appears to wrap around objects and produces hazy, indistinct edges. Strong illumination is suggested by diffusion.

The visual language is full of contrasts: light and dark, warm and cool, strong color versus neutrals, flatness and volume, etc. Visual opposites accentuate one another; each makes the other appear more distinct and exciting. Specific to luminosity, there are two visual pairs that may be played off each other:

- VALUES - light vs. dark
- CHROMA – high chroma vs. low chroma & neutrals (white, black, brown, grey)

These contrasts are part of the transition from light to shadow on three-dimensional forms (as discussed above). However they also can be considered in a larger context, within a painting as a whole.

For example, a fair-skinned portrait appears brighter (i.e. more luminous) set against a dark (as opposed to a light) background. A bright, yellow lemon seems more brilliant atop a dull, brown tabletop (versus on a chromatically intense surface). In short, the greater the value and chroma differences between a light, bright color and what surrounds it, the more brilliant and luminous that color appears.

The methods above (a light source and contrast) "suggest" luminosity; they create an illusion of illumination. It is also possible to impart actual, luminous effects to physical layers of paint by...

The luminance of a color is determined by four factors:

- Hue. Some hues have inherently greater luminance. Yellow has the most, followed by yellow-green, pale blue, pink, orange, red, purple and blue (for a color luminance chart, see ).

- Chroma. The more pure and intense a color is, the greater its luminance. As chroma is lowered, luminance generally decreases.

- Value. The lighter in value a color is, the greater its luminance.

- Contrast. As described in #2 above, the greater the value and chroma differences between a bright color and the colors that surround it, the more luminance the bright color appears.

If you apply transparent color over a white, opaque ground or white, opaque underlying paint layer, some percentage of light travels through the transparent color, bounces off the opacity, and is reflected back - the surface literally radiates light. There are a few things to keep in mind when applying glazes:

- Underlying value and opacity. The more white and opaque the underlying surface to which a glaze is applied, the more light is reflected back.

- Attributes of the color used to glaze. The greater the inherent luminance and transparency of a color, the more luminosity it imparts as a glaze.

- Number of glazes. White is the lightest value pigment. Yellows are generally light; most other colors are mid or dark values. So the more glazes that are applied (particularly mid to dark value ones), the darker and more obscured the white ground becomes. In short, too many glazes at some point decrease luminosity. It may become necessary to reintroduce a layer of white (a “scumble”) over parts or all of the painting before applying more glazes.

As the above indicates, there are several ways to create luminosity in painting. Not all are requisite within a single painting. This is why very different types of painting each may be called luminous. Botticelli (1445-1510) painted in egg tempera with a primarily high-key, relatively high chroma palette and subtle light effects; his images have a soft, luminous glow. Rembrandt (1606-1669) painted in oil using mostly low key, low chroma colors, with dramatic light and shadow effects; his work appears radiant with light.

Egg tempera is often considered an especially luminous medium. Given the thinness of the paint, its resulting transparency, the many layers of glazes and scumbles that can be applied, and the reflectivity of a white gesso ground, tempera paintings often do take on a radiant glow. However tempera painters may want to keep in mind the other methods through which luminosity is conveyed (imply a light source, create contrast). It's important to understand that any medium can appear more or less luminous, depending on how attentive a painter is to the various ways in which luminosity is achieved.

HOWEVER…Don’t Confuse the Means with Ends
Once you understand how luminosity is conveyed, it may be tempting to focus exclusively on achieving it. A great painting is like a perfect, visual “ecosystem” in which the visual language is arranged in such a way that every element has its reason for being there, and each is in balanced relationship with the whole. Light, values, colors, shapes, line, edges, subject matter – all must be considered. Luminosity can contribute beauty to a painting but is not requisite; it is just one brilliant piece of the puzzle.
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Old 15-01-18, 08:43 PM
Koo Schadler's Avatar
Koo Schadler Koo Schadler is offline
Tempera Painter
Join Date: Dec 2003
Location: Alstead, NH & Zirahuen, Mexico
Posts: 318

Below is a glossary that accompanies the handout...


Many disciplines study light and color – physics, chemistry, psychology, history, and art, to name a few. Each may have differing definitions of identical terms. The modern, scientific understanding of light and color may help or hinder an artist. Along with greater knowledge comes the risk of unnecessarily complex science that – to a painter - may not be relevant. After all, the Old Masters had no “colorimeters” or “spectroradiometers” yet they used light and color brilliantly.

This glossary is not definitive or the only way to understand the terms. My goal is to offer definitions that are accurate (relative to painting), not unduly technical, and helpful to achieving luminosity.

Bright refers to how much light appears to radiate from a surface. Brightness is a subjective impression of luminance.

The term bright describes an appearance of luminosity relative to both value and color; however it also can be used to describe value only. For example, a high value, high chroma yellow may be called bright. A high value, no chroma white may also be called bright. However some would say white more accurately should be described as “light” (not bright).

This subtle distinction serves a practical purpose. If a painter wants to make an image appear “brighter,” what specifically does that mean – increase chroma, lighten values, or both? When defined imprecisely, the word bright can confuse rather than clarify - hence the distinction between a “bright” yellow and a “light” white.

The vividness, purity, intensity, or strength of a color. High chroma colors are colorful. They are referred to sometimes as saturated or fully saturated.
Low chroma colors are dull, dirtied, muted, washed out, or near neutral. They are referred to sometimes as de-saturated or unsaturated.
Adding white, black, brown, grey, or a color’s complement to a color lowers the color’s chroma.

A thin, transparent film of paint applied over a painting. Pigments that are transparent are best for glazing. However almost all paints, if sufficiently dispersed (by their thinner) and thinly applied, can be made to appear transparent.

The transparency or opacity of a pigment is largely determined by the refractive index value (RIV) of the pigment minus the RIV of the medium in which the pigment is bound – this means a pigment’s transparency/opacity can change depending on the medium it is in. A pigment’s particle shape and size, as well as pigment volume concentration (PVC) also influence transparency. Despite variations due to the above, most pigments tend to characteristically appear either transparent, semi-transparent, or opaque .

The basic color family (red, yellow, blue, green, purple, orange) to which a color belongs. For example cobalt is a blue hue, alizarin a red hue, viridian a green hue.

Paint that is applied very thickly; three-dimensional paint.

A color’s value. How dark or light a color is.
A high key painting has a majority of middle to light values.
A low key painting has a majority of middle to dark values.
Note that key is distinct from chroma: how light or dark a painting appears is not the same as how colorful or dull it is. For example, a painting of intense red, green and blue is high chroma, but not high key; it is low key.

The source of illumination on an object.

The perceived color of an object unaffected by light or shadow. For example, the local color of grass is green.

The perceived brightness of a color.

Radiating or reflecting light. Bright, shining, radiant.

When an object is illuminated by a single light source.

A thin layer of white paint applied semi-transparently over other colors, like a veil or mist. The white may be tinted with a bit of color. Even though whites are opaque to semi-opaque pigments, white paint can be applied thinly or dispersed enough to behave (appear) semi-transparent. (A second, more commonly used definition of the word scumble is a thin, brushy application of paint using a well-wiped brush.)
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