Various units used in measuring the amount of solar energy falling upon a surface, used, for example, in designing solar water heaters. Abbr, ly. It is named for Samuel Pierpont Langley (1834 – 1906).

In some early work it is a unit of power, cal₁₅ per square centimeter per minute (approximately 698 watts per square meter). Later it is defined as calories per square centimeter (“surface density of radiant energy”), but the precise value depends on which calorie is used. One langley can be:

- 1 calorie
_{IT}per square centimeter, approx. 41,868 joules per square meter - 1 calorie₁₅ per square cm, approximately. 41,855 joules per square meter
- 1 calorie
_{th}per square cm, approx 41,840 joules per square meter

The last is most common.

According to the current national standard in the United States¹, the langley is not to be used. Joules per square meter should be used instead.

1. IEEE/ASTM SI 10™-2002.

*American National Standard for Use of the International System of Units (SI): The Modern Metric System.*

New York: IEEE, 30 December 2002.

See Section 3.3.3.

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Last revised: 30 May 2007.