Heat Radiation Calculation

Not sure what you mean by heat radiation "zone" when you only furnish overall heater surface dimensions and the words "concave type"

Assuming that the concave ceramic heater surface has an approximate focal point, and that the "focal" point is 200mm in front of it you can use standard black body radiation/absorption formulas with their appropriate constraints to obtain an approximate temperature at that point. Keep in mind that these calculations assume a vacuum, and will be closer to measured values at higher temperatures, all other things being equal. Be prepared also for discrepancies in any real measurements from the calculated "ideal" values, depending on the specific materials and means and method of the measurements. The concept of black body radiation and the formulas involved are valid, but their accurate application can be tricky in practice (especially for the uninitiated) for the calculation you seek, unless you just want a comparative guide.

Typically, in a well executed experimental setup, you would establish a correlation of calculated and observed temperature values on your specific ceramic heater surface and target, then having done this, can go on with some degree of confidence to rely on this model to extrapolate from purely calculated values for the rest of your data points.

Lacking any "focal" point, and/or measuring the achieved temperature over an area larger than an idealized point, you would still use those formulae, based on the actual measurements involved.

You can find much information on the web. This link may be of help:

http://en.wikipedia.org/wiki/Blackbody_radiation#Equations_governing_black_bodies

If you need "real" numbers, then you have to know the temperature of the surface of the ceramic heater element, in which case you need something to measure it with. This then begs the question of why not do your experiment empirically, and measure the temperature of your target also?