light, colour & photography

light value (Lv)

you might be interested in these articles: film speed | shutter speed | aperture & stops | weston master v

then you might want to read the next article: exposure value

This article should be read in conjunction with "Exposure Value", a term with which it is often confused.

Our everyday experience is one of looking at and seeing our environment. We are aware of bright objects, emitters of light such as the sun and artificial light sources. We are also aware of bright objects that do not by themselves emit light but reflect light. We can call them secondary sources.

Secondary sources are important in photography because they also contribute to the illumination of a scene both in terms of the amount of light they reflect as well as its colour (in photo-realistic rendering this is known as "radiosity"). We use radiosity quite consciously when we use a white, silver or gold reflector to add fill light to a subject. No matter how white an object's surface it is self-evident that without any light source, nothing - not even the whitest white - can be seen.

The distinction between the amount of light that fills space and the objects illuminated by that light is an essential one to grasp. We never see the light itself (unless we look directly into a light source) but are aware of its presence by virtue of the objects it illuminates. We gain an impression of the amount of light by how bright the objects themselves look as well as in relation to each other.

By way of example we look at a cloudy sky. When we see white clouds illuminated by the sun; they they become secondary sources. When they obscure the sun and appear grey they reduce the illumination and hence the brightness of a scene. Cloudscan therefore mask and alternately contribute to the light falling to ground. On a cloudless day or one with white cumulus clouds the impression is of a bright day; when the sky is overcast the day is characterised by being "dull" or "dark".

It is difficult for us to be exact in our impression of the amount of light that fills space. The eye is an organic optical system, not a mechanical one, so our impression of brightness varies from person to person and varies over time. Coming from a dark room out into the sunlight makes the outdoor scene appear brighter than it really is; the converse is that after entering a room from bright sunlight we have to wait a moment for our eyes to get used to the dark [insert footnoote]. Not so for measuring instruments - the amount of light can be measured instantly and accurately. It is the science of photometry.

The subjective impression of the amount of light that fills space is called "Light Value". The phrase was first used by Ansel Adams in his 1948 book on

The amount of light filling space as measured by an instrument is called "illuminance". In its most fundamental sense, Light Value has nothing to do with photography nor technology nor measurement but has everything to do with how we experience our environment - specifically how bright our surroundings appear. This is the primary characterization of "Light Value"

But wait a moment... If our surroundings appear bright, surely there is more light filling the space we see? And if Light Value is to do with the amount of light filling space then surely we can measure it? Well yes we can - and this would be a secondary characterization, a construct for which we would need a scale of measurement.

light value scale

The human eye can distinguish the faintest stars to the brightest natural light we know of - sunlight. This is an enormous dynamic range and one that is possible only because the eye can adapt to the surroundings by changing pupil size and the rate of recovery of Rhodopsin within the retina. The range is approximately 10⁹ (1,000,000,000)

In practice the dynamic range is always less than this because the eye cannot extend its range at the brightest levels whilst at the same time remaining sensitive to the lowest levels. For the most part we exist in a comfortable range between bright sunny days and the sort of levels we encounter at night-time in our streets and urban areas. This is a dynamic range of about 1:500,000; if we disallow dark adaption the range is much more like 1:16,000.

We can represent this range numerically. If we choose a linear scale we will find that all the interesting stuff is squeezed into the lower end of the scale. If we choose a logarithmic scale it will even out the perceptual differences between light and dark. If we use base 2 for the logarithmic scale then equal perceptual differences can be represented by a doubling of the Light Value.

The scale of Light Values is shown in the following example. The first column has a scale ranging from -2 to +24 but it should be noted that the scale has no theoretical limits. Practically, nothi

Table 1: Light Value (Lv) and Subjective Impression of Brightness

thumbnail	description
	Pop-up JPEG version of the table (reduced size)
	Full PDF version of the original. (Opens in new window or tab) Please refer to the following terms & conditions for restrictions on copying and distributing the chart: The values shown on the chart are normalised for ISO 100 film speed.

You will find many web sites and reference sources where a similar diagram is shown - the difference being that the scale is shown as "Exposure Value" (Ev) and not Light Value (Lv). The failure to make the correct distinction between the two has caused endless confusion. Hopefully that confusion ends here.

The Light Value scale allows us to put a number against particular light levels. Remember that every description in this table relates to a specific experience. The table attempts to place a quantity (a number) against a particular experience. Experiences are notoriously difficult to pin down however; we all know what is meant by "brightly-lit street scene" but is the brightly-lit street outside my home the same as the street outside your home?

Although the table shows a scale of numbers, those numbers are no more precise than the descriptions themselves. If we want to be more precise we must move away from experience and turn to measurement.

the light meter

Our experience of the amount of light filling space depends on a physiological reaction to light. It is an organic system and, in common with almost all other organic systems, what has gone before has a profound effect on what we experience now.

A light meter relies on a purely physical reaction to light' the most useful of which is a change in an electrical property. The electrical response can be one of changing resistance to an electrical current or one of generating electrical energy directly from the light energy received. [article required: light meters]

A system of numbers linked to a quantifiable electrical response means that a light meter can be calibrated to an appropriate scale. Although Light Value scales can be completely arbitrary; a meter can be calibrated to show values similar to those shown in the above table.

summary

Here is a summary of what Light Value is not:

• it is not Exposure Value
• it has no units
• it is not part of the SI system and does not derive from any agreed standard

Here is a summary of what Light Value is:

• it is an experience
• it can be described in subjective terms
• it can be measured using photo-sensitive materials and equipment
• it can have almost any scale you want but the scale based on Log₂ is the most useful
• it can be related to Exposure Value - but only in the context of light-sensitive materials that can be exposed to light; that is, film speed

The relation between Light Value and Exposure Value is discussed in the article exposure value.

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