There are factors that come into play that make the above statement incorrect. An example would be when heating white plastic sheet, as in a thermoforming application. Short wave infrared is color sensitive. Meaning that black colors will quickly absorb short wave infrared, but white colors will reflect a much larger amount of the energy. Therefore, when heating a white sheet of plastic, the medium wave can actually heat the sheet faster than the short wave emitter.
Testing is the only way to truly determine the results.
All of the heaters in the market do not output energy at a single wavelength. Many factors will come into play when making the decision on which wavelength emitter is appropriate. Answer Infrared heat does penetrate the sheet within limits. It has been stated that infrared energy will penetrate the sheet up to .015 inches. Past that point the sheet is heated via conduction.