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Infrared radiation heating is becoming widely applied to drying processes of wet industrial materials and to heating processes for food-stuff containing water etc. Thus a growing importance is attributed to clarify the exactpenetration depth of IR radiation into water from heating engineering view point. Many IR transmittance data on water film have been published by various authors. All of them as far as the author knows, however, failed to indicate the detailed optical data of the two window plates used to fix water film thickness. Accordingly the exact penetration depth (or inverse of Lambert absorption coefficient) into water cannot be known, since multi-reflection takes place within bounded water and window plates too, thus affecting the transmittance value to be measured. Moreover in the measurement of very thin water film, there takes place often an interference between forward going electro-magnetic wave and successively reflected backward one. This too affects the measurement particularly in IR region of higher transmittance. Messrs. Robertson & Williams tackled with the captioned theme in the early years, and indicated, in their report"; difficulty to remove the unfavorable effects introduced by window plates in view of finding Lambert coeff. of water. The difficulty had been experienced those days by many authors. Robertson & Williams elaborated a special precise absorption cell with window plates of either CaF2 or KRS-5, and succeeded in giving fairly good Lambert coeff.∝ for 2.33 - 33.3 micro-meter wavelength region. They seemed to have paid enough experimental consideration and made some corrections on the inevitable interference effect, but not enough on the multi-reflection effect. Accordingly their derived formula for giving∝ was not fully theoretically correct, since taking ratio of the two transmittance values with different water thickness did not cancel out all of the window plate effect. The author has carefully taken into consideration the above two effects in his analysis and measurement. He first established analytical formulae comprizing multi-reflection effect under condition with no interference, and second rejected to employ measured transmittance data seemingly under interference state, thus has succeeded in giving the exact penetration depth for 0.9 - 25. micro-meter wave with normal-incidence to free water surface at room temperature(25 ± 5°C). Conversely, taking advantage of the mentioned unfavorable interference, he has also succeeded in giving water refraction-index spectrum covering 0.9 - 2.5 micro-meter region. The same spectrum for 2.5 - 25. micro-meter is also given in the text, by using a special reflectance measurement, though the latter being approximate value. The above water refraction-index spectra are, depending on wavelength region, in good or poor agreement with the corresponding table data which was compiled by Messrs. Hale & Querry in early years from those days published date The spectrum data obtained by himself was employed in later calculation to find 0C.
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