Watt per Square Centimeter [W/cm²]

The unit watt per square centimeter (W/cm²) measures heat flux density or radiant flux, representing the amount of energy transferred per unit area per unit time, with the area measured in square centimeters. One watt equals 1 joule per second, so 1 W/cm² corresponds to 1 joule of energy passing through 1 cm² every second.

This unit is often used in high-intensity applications such as:

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  Laser systems measuring power density on a target

  
  
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  Plasma or arc discharge studies

  
  
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  Industrial thermal processes where energy is concentrated on small surfaces

  
  

The relationship can be expressed mathematically as:

where q is heat flux in W/cm², Q is energy in joules, A is area in cm², and t is time in seconds.

Conversion to SI units:

While W/m² is the standard SI unit for most applications, W/cm² is convenient for small-area, high-intensity measurements, such as in laboratory-scale experiments, laser technology, and materials testing, where energy is concentrated over very small surfaces.

Btu (th) per Second per Square Foot [Btu(th)/(s·ft²)]

The unit Btu (th) per second per square foot, written as Btu(th)/(s·ft²), measures heat flux density or thermal power per unit area using the thermochemical British Thermal Unit (1 Btu(th) = 1054.35 J). It represents the amount of heat energy transferred through 1 square foot of surface every second.

This unit is commonly used in high-intensity heat transfer applications, industrial processes, solar energy studies, and material testing, especially where energy flux over relatively large surfaces is significant. Applications include:

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  Heat flux from furnaces, radiators, or heaters

  
  
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  Solar radiation received by panels or flat surfaces

  
  
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  Thermal testing of building or industrial materials

  
  

Mathematically, heat flux is expressed as:

where q is heat flux in Btu(th)/(s·ft²), Q is energy in Btu(th), A is area in ft², and t is time in seconds.

Conversion to SI units:

Although W/m² is the SI standard, Btu(th)/(s·ft²) remains useful in U.S. engineering, thermal studies, and historical literature, providing an intuitive measure of high-intensity heat flux per square foot per second.