What is a perfect black body


The change in emissivity of a black body or between different black bodies (at the same temperature) can also be formulated accordingly:

T ‘is the effective temperature corresponding to the radiation in the sphere opening of the black body, which is measured by the probe at the same ambient temperature TA. The temperature of the measuring probe must be taken into account, as the difference between the emissivities TA -> TBB ‘cannot be determined.

Black body with certifiable emissivity

The shape of a spherical cavity was chosen for the certification of the blackbody (BB) because it is the only geometry for which the emissivity can be calculated [1].

The effective emissivity - determined using the Bedford method - can easily be looked up for a spherical structure in [1]. With a ratio of opening radius to spherical radius of Ra / Rs = 0.1 and an emissivity of the inner wall surface made of anodized aluminum of εW = 0.9, the effective emissivity is εBB = 0.9997. We can use this value as a basis to estimate errors in the tolerance range 0.7 <εw <0.9 to 0.9989 <εBB <0.9997. The emissivity of inner walls made of anodized aluminum was estimated by [2] and was determined precisely experimentally using a conical reflector [3]. This resulted in values ​​in the range εw = 0.9 + 0.02 and εw = 0.9 - 0.1. Accordingly, the emissivity can be calculated directly from equation 8:

0.8 <εw <0.92 => 0.9993 <εBB <0.9997

All other conceivable sources of error, such as deviations from the spherical geometry or surface disturbances along the seam between the two spherical halves, are plausibly negligible. Errors due to temperature fluctuations in the water bath, which ensures that the entire sphere is at the same temperature, are independent of the sphere geometry and are also considered to be negligible if the immersion bath is properly insulated.