RADIATION OF HEAT FROMTHE HUMANBODY. V. THETRANSMISSIONOF INFRA-RED RADIATION

THROUGHSKIN

By JAMESD. HARDYAND CARL MUSCHENHEIM(From the Russell Sage Institute of Pathology in affiliation with the

New York Hospital, New York)

(Received for publication August 9, 1935)

In 1934 the authors (1) reported the results ofspectroscopic observations on the emission, reflec-tion and transmission of infra-red radiation by thehuman skin. The experiments were undertakenwith the primary purpose of determining -whetherthe emissive and absorptive properties of the skinwere those of black-body radiator within the spec-

tral range in which the human body radiates heat.The results of the experiments, carried out by theuse of a reflecting infra-red spectrometer equippedwith a rock salt prism, confirmed previously ac-

cumulated evidence (2, 3, 4) as to the truth ofthis assumption, and thus seemed to establish be-yond all doubt the validity of the radiometricmethod of skin temperature measurement. Re-cently published results of Christiansen and Lar-sen (11), using questionable technique, wouldlead one to the dubious conclusions that the whitehuman skin has a radiating power only about 78per cent that of a black body and that the skin isquite transparent to the infra-red radiation in theregion of the radiation emission from the humanbody. This new evidence is in direct contradic-tion to that of the combined work of nearly allother workers on this problem; it will be brieflydiscussed below. During the course of our previ-ous transmission experiments it was noted that:(1) the penetrability of skin for the near infra-red region of 0.75 ,u to 3 u, which is not includedin the range of black-body emission above alludedto, was considerably lower as found by us than as

reported by the majority of previous observers(5, 6, 7, 8, 9, 10) and (2) the transmission spec-

trum of the thinnest layers of skin showed an ap-

parently characteristic fine structure which re-

sembled that of the infra-red spectrum of many

organic compounds, and its most prominent bandswere thought to be due to the C-H, N-H and0-H linkages in the organic substances compos-

ing the skin.The present report is of further experiments

concerned principally with these two findings ofthe former investigation. Since nearly all ob-servers agree that it is only within the near infra-red range than any appreciable penetration of theskin occurs, it is this portion of the spectrum, ifany, which is effective in the deep penetration ofradiant heat. As the present use of infra-redtherapy is based partially on the supposition thatthe infra-red radiation penetrates in effectiveamounts, it was thought desirable to repeat theobservations on living skin as well as on deadskin, on which alone our previous observationshad been made. To determine more exactly theamount of scattered transmitted radiation, thusdetermining accurately the total amount of trans-mitted radiation, is also of importance. Thisrange of wave lengths also includes those effectivein infra-red photography, and the question of howfar and in what quantities the radiation penetratesis of importance in determining the limitationsof this method of photographing subcutaneousstructures.

As regards the second of the findings, i.e. thepresence of characteristic absorption bands in thetransmission spectrum of very thin (epidermal)samples of skin, it seemed desirable to repeat theseobservations also, using an instrument with greaterdispersion, and thus to resolve the absorptionbands more exactly and possibly identify them.

METHOD

The present set of observations were made by means ofa rock-salt prism spectrometer designed by one of us andbuilt in our laboratory by Mr. G. F. Soderstrom; its de-sign and construction will be reported later. The chiefdifferences between it and the instrument used in theformer investigations are that a larger prism was usedand that the light beam was made to traverse the prismtwice. Both of these modifications produced a far greaterdispersion and consequent better resolving power. Cali-bration was carried out with great care using some 23calibrating points, and is considered correct to .01 S. As

1

JAMES D. HARDYAND CARL MUSCHENHEIM

in the former experiments a Nernst glower was used asthe energy source and a vacuum thermocouple as the re-ceiving element. A Leeds and Northrup high sensitivitygalvanometer was used in conjunction with the latter, andthe deflections were read directly on a glass scale. Apotentiometer was connected in series with the galvanom-eter, and was used whenever the deflections were verylarge, thus obviating the necessity of changing slit widthsbetween measurements of the direct and transmitted beamwhen these differed greatly in energy. The slit widthsused varied from 0.1 to 0.5 mm., and the data presentedare without slit-width corrections.

3

A and the diaphragm B being known The value thuscalculated may then be applied as a correction factor forscattering to the direct transmission values.

Observations both of direct transmission and scatteringwere made on (1) relatively thick specimens of dead hu-man skin obtained from surgical amputations, (2) the en-tire thickness of a rabbit's ear first alive and later ampu-tated, and (3) thin epidermal pieces of human skin ob-tained by cantharides blister from the volar surfaces ofthe forearm of living subjects. As it was found inpreliminary experiments that the method of sponging thedead specimens with saline to prevent excessive drying

N

FIG. 1. DIAGRAMOF EXTERNALOPTICAL SYSTEM.

The external optical system was essentially the same as

used in the former investigation and was a modification ofa method originally developed by Hutchins (12) for themeasurement of scattering. The arrangement for this isshown in Figure 1. The specimen to be examined ismounted over a rectangular diaphragm at A, so placed as

to be normal to the axis of the cone of incoming rays.

The light from the Nernst glower is focussed upon thespecimen so that at A we have a source of radiationwhich emits in all directions, depending upon how muchscattering takes place. The light source N and the con-

cave mirror Ml together with A can be rotated about a

vertical axis through A by any desired amount. Asecond rectangular diaphragm B is fixed with respect tothe mirrors M2, Ms and the entrance slit of the spectrome-ter S. By rotating the specimen with the light source

*the distribution of the scattered light with the angle maybe determined, as was done in the former experiments.By the interposition of the diaphragm B, however, it ispossible to go further and integrate over the entirehemisphere the total amount of scattered radiation fromA, the distance R and the dimensions of the glower image

was not entirely effective in preventing drying and conse-quent change in penetrability of the tissue, this methodwas abandoned, and the entire specimen with apertureframe was kept immersed in the normal saline betweenreadings. This was made possible by having the frameslide in a grooved holder which was accurately fitted sothat the skin specimen always returned to the same posi-tion. As a result, the skin was exposed to the radiationonly as long as it took the galvanometer to come toequilibrium-about 7 seconds-which was not long enoughto cause any drying or change in penetrability. The liv-ing rabbit's ear could uot be conveniently attached to thesliding frame so that the latter was left in place in theholder, and the ear held against it for each observation,the placement being oriented by means of the vascularmarkings of the ear. The living ear was not artificiallymoistened as was the dead tissue. It had been shaved ofall hair before the experiment. The thin human skin ob-tained by cantharides blister was examined in the moistcondition as above described and again after having beenthoroughly dried. The reason for this will appear in thediscussion of the results. The dried skin was placed be-

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HEAT RADIATION THROUGHBODY -

tween two rock salt plates to prevent overheating andscorching. The procedure in general was to determinethe direct transmission by measuring the intensity of theincident beam and of directly transmitted beam at eachspectrometer setting. For the total transmission theamount of energy scattered from the normal must betaken into account. Accordingly, with the spectrometerset for a given wave length, the specimen with its opticalsystem described above, was rotated about a vertical axisby 5° steps. Observations were made with increasingangle until the deflection reached zero. In this mannerthe form of the distribution of scattering about the angle,9 = 0, was determined. Measurements of scattering weremade at I I, 2i., 3k', SiA, and 7,', and as the scatteringvaries only gradually with wave length, becoming lesswith greater wave lengths, the corrections for inter-mediate wave lengths were interpolated for over a con-siderable range. The error thus introduced is negligible.

The correction for scatteringFrom a consideration of Figure 1 it is apparent

that what is being measured, for any value of 0, is

the distribution of scattering over the hemisphere,mathematically, if the function is easily deter-minate, or by graphical integration. Figure 2shows such scattering curves for several materialsmeasured at 1.7 ju, and it may be seen that whilewhite bond paper follows the simple cosine func-tion of Lambert's Law very well the other. mate-rials, including those with which we are concerned,do not. Angstrom's modification of Lambert'sLaw was found to fit the data for ground glassand skin so that the integrations, whether graphicor mathematical, were in complete agreement ifthis function were used.

When the total scattering is thus determinedby integration of the distribution curves, thetotal energy transmitted, I, is given by theequation

I= BB1=2 wR-P Sf

l=

S=zrf () SIN eaeLAMBERTaS LAW

f(O3)ZE.COSANGSTROMSLAWA cose-eAI) siN

0

A NGL EFIG. 2. DISTRIBUTION CURVESOF SCATTEREDTRANsMITTEDENERGYFOR VARIOUS MATERIALS.

the energy emitted from the surface A and re-ceived by the surface B. By measuring thisamount at various values of 6 the angular distribu-tion curve of the energy scattered over the hemi-sphere may be determined. The total energy canthen be calculated by integrating the function of

in which S = total scattering = o2V2f(O) sinOA&O.

For Angstrom's law

=(o)=.cos 'f()1 = (,u2 _ 1) sin 2 O'

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JAMES D. HARDYAND CARL MUSCHENHEIM

where 0 = angle from the normal,EO = intensity of the scattering for 0 = O,

f(0) = intensity of the scattering for anyangle 0,

ju = constant.All of these quantities except Iu are measurable

and therefore , can be determined.R, F, and B are all determined from the experi-

mental set up:

R = distance between diaphragms,F = obliquity factor

1

-_ (a2 + a2 + b2 + #2)4R2

B = area of receiving aperture whose dimen-sions are a, b,

a, , = dimensions of emitting aperture A.

The percentage of the total incident energytransmitted, T, is determined by the ratio of thetotal amount of energy transmitted to theamount of incident energy Io, or

IT =

I

Observations were made on ground glass asa means of checking the method. The total trans-mission through a plate of ground glass wasdetermined and compared with the direct trans-mission through a plate of polished glass cut fromthe same piece. It was found that the polishedglass transmitted 68 per cent and the ground glass61 per cent. A discrepancy of about this mag-nitude is to be expected because of internal re-flection at the ground surface so that the groundglass should transmit somewhat less than theclear specimen and the agreement must be con-sidered very good.

In our earlier communication (1) on infra-redtransmission of skin it was pointed out that theattempts of other investigators (9, 10) at correct-ing for scattering by the use of methods whichinvolve placing the specimen in close proximityto the receiving thermo-element introduce theerror of reradiation. In this way they were ledto suppose that the scattering of the transmittedlight followed Lambert's Law, but as is shownhere, such is far from the case with skin or evenground glass.

RESULTS

In Figure 3 are shown graphically the resultsof the experiments on transmission through arabbit's ear and through two relatively thick speci-mens of human skin. The thicknesses were de-termined by measurement of sections mounted onmicroscopic slides after routine fixation, section-ing, and staining following the termination of theexperiments. All of the curves are corrected forscattering except Curve 3a which corresponds toCurve 3 uncorrected.

Curve 1 represents the transmission of the liv-ing rabbit ear. Curve 2 represents the transmis-sion of the same ear a few minutes after beingsevered from the rabbit. Curve 3 represents thetransmission of the ear after it has been dead fortwo days. The thickness of the ear was 0.65 mm.

The two pieces of human skin were of 1.4 mm.and 2.0 mm. maximum thickness respectively.Both of these were obtained from an amputatedleg. The thinner piece was filed down to quiteuniform thickness after freezing. The thickerpiece was neither frozen nor filed but merelytrimmed down with a razor blade as well aspossible to a fairly even thickness. The filing andtrimming, of course, was done on the inner sur-face, and the material removed was merely sub-cutaneous fat and connective tissue. Both speci-mens were actually composed of skin plus somesubcutaneous connective tissue, the skin itselfforming less than half of the total thickness.They were kept in normal saline solution in theicebox until examined, the thinner piece one dayafter amputation, the thicker not until two daysafter amputation.

Examination of the curves for the rabbit earreveals that the living tissue allows considerablyless penetration than the dead and that, moreover,the longer the tissue has been dead the greater thetransmission. Comparison of the uncorrectedcurve (3a) and the corrected curve (3) showsthat a large proportion of the energy is scattered.This accounts for the much greater transmissionthrough human skin shown here than we reportedfor comparable thicknesses in our former investi-gation, since in the latter 6nly the direct trans-mission was measured. Thus we find a maximumtotal transmission of direct plus scattered radia-tion of 10 per cent for a thickness of about 1 mm.

4

HEAT RADIATION THROUGHBODY

while the direct transmission alone amounts toonly about 1 per cent. Even the corrected value,however, is very much less than the 21 per centmaximum transmission through 5 mm. reported

a thin piece of skin consisting of the horny layerof the epidermis alone, .03 mm. in thickness.The absorption bands in the curve for the wetskin are all identified as being due to water, in

RABBITS EAR0.65 MM.THICK.

1.4 2.0 2.4 ,hWAVELENGTH.

F

HUMANSKIN.

FIG. 3. TRANSMISSION CURVESOF RABBIT'S EAR AND OF THICK SPECIMENS OF HUMANSKIN. CORRuCTEDFOR SCATTERINGExcEPT CURVE3a WHICHREPRESENTSCURVE3 UNCORRECTED.

Curve 1-living rabbit ear.

Curve 2-rabbit ear dead one day.Curve 3-rabbit ear dead two days.Curve 4-human skin, thickness 1.4 mm.

Curve 5-human skin, thickness 1.5 to 2.0 mm.

by Danforth (7) and Cartwright (8). From our

results, the estimated maximum transmissionthrough such a thickness would be less than 1per cent. If living skin were used the transmis-sion would be even less as the rabbit ear experi-ment shows. The absorption bands found in thecurves may be identified as due to water, as alsois the cut off at 2.4 ,. It will be noticed thatthey are present in the curve for the living rabbitear which was not moistened with saline, as wellas in the other curve.

In Figure 4 is given the transmission curve of

fact the curve is an exceptionally good exampleof the water spectrum. The curve is correctedfor scattering but no correction is needed beyond7.6 t From Figure 2 it is obvious that the mag-

nitude of the scattering factor is very much lessfor such thin epidermal pieces than it is for thethick specimens. The intense water spectrum ofthe wet skin has evidently obscured the bands dueto other components of the skin. On drying, how-ever, bands due to other radicals appear whilethe water bands become less prominent or even

disappear entirely. Thus in the curve for dry

z0

20 .In

24.:

5

JAMES D. HARDYAND CARL MUSCHENHEIM

skin we find that a band has appeared at 3.45,umwhich is due to the C-H bond and that the 4.7,uband is no longer recognizable. Shifts have alsooccurred in the double band at 3 , and the doubleband at 6 ,u in which the N-H and C-H bonds areboth concerned.

thermocouple to measure the absolute skin tem-perature has been criticized before and any con-clusions drawn from data thus secured have beenshown to be untrustworthy. Further, these au-thors have not explained the low reflecting powerand transmission of the skin to the long infra-red

HUMAN EPIDERMIS .03 MM.THICK.

6 7 8WAVELENGTH.

FIG. 4. TRANSMISSION SPECTRUMOF HUMANEPIDERMIS, WETAND DRY.

Observations on the infra-red spectrum of otherbiological materials are being made since it seemspossible that infra-red spectroscopy may be apowerful potential tool for biochemical analysis.

DISCUSSION

The results of the present investigation confirmthe conclusions drawn earlier as to the radiatingcapacity of the skin. That is, the skin in appre-ciable thickness is quite opaque to radiation ofwave lengths greater than 3 p whether the tissuebe living or dead. This is in contradiction to thefindings of Christiansen and Larsen. These au-thors, using the Cobet radiometer and a surfacethermocouple, concluded that the radiating capac-ity is 78 per cent that found by us, and that theradiation beyond 4 u penetrates the skin in ap-preciable amounts. The technique of using a

as was found by us. Also the whole of the evi-dence, with this exception, radiometric and spec-troscopic, seems to be in complete agreement inplacing the radiating capacity of the skin at about99 per cent that of a perfect radiator.

The quantitative method of integrating the scat-tered light transmitted through the several layersof skin permits the estimation of the total amountof energy penetrating one layer and incident onthe next. The striae of the tissue act as ref ract-ing, reflecting and scattering areas: thus the lightwhich comes through a layer may possibly havetraversed a thickness greater than that of theactual layer. There is no way of determining theexact path of a given ray and it is not possible,therefore, to compute the absorption coefficient ofskin in the true optical sense. One can, however,compute what may be termed the " effective " ab-

6

HEAT RADIATION THROUGHBODY7

sorption coefficient which has been found to be5.5 for the corneum and 1.54 for the lower layersand subcutaneous tissue for XA 1.2 ,.

:z 2Ii;0 4° X--

-LJ

cc F- i--LLI--

co Xr 9n

100

:i 800

-60

Liiz

a. 40

2t

20

0

cutaneous tissue; 99 per cent of the total radiationis absorbed within 3 mm. of the surface. It isevident from Figure 3 that for good penetration

0 1 nM. 2 mm,

THICKNESS.FIG. 5. SCHENIATIC DIAGRAMSHOWIINGEXTINCTION\ COF INFRA-RED RAYSOF THE MOST

PENETRATINGWAVELENGTH (X = 1.2 A) BY DEADTISSUE.The blood in living skin would cause a much more rapid extinction.

If one chooses the most penetrating of all the of the infra-red rays, a lamp of very high tem-infra-red radiations, wave length = 1.2 ,x, a curve perature should be used so that the maximum ofof maximum penetration can be drawn as is shown radiation shall fall at X = 1.2 p. The tempera-in Figure 5. ture of such a lamp is easily calculated from

Of the total incident energy, 21 per cent is Wien's Law.reflected by the surface; 66 per cent penetratesthe corneum; 50 per cent penetrates to the sub- Xmax T = 2940,

7

JAMES D. HARDYAND CARL MUSCHENHEIM

where T -absolute temperature of the lamp andAmax== wave length of maximum radliation in u.

Thus for Anmax =1.2 Lu,T =- 21000 C. as the optimal lamp

temperature, a temperature easily reached by abright tungsten filamenit.

As the infra-red radiation is wholly absorbednear the surface by both living and dead tissue, thetherapeutic benefits from such rays must be pro-vided by some peripheral mechanism. The ther-mal effect of these rays is well known, but inas-much as 50 per cent of the radiation passes

PIATE I

PLATE II

FIG. 6.

PLATE I. PHOTOGRAPHOF VOLAR SURFACE oF AR-M WITH EASTMAN PANCHROMATICPLATE; NOFILTER.

PLATE II. PHOTOGRAPHOF SAMESURFACEWITEI EASTMANPLATE I-R USING INFRA-RED FILTER WHICHWASCOMPLETELYOPAQUETO VISIBLE LIGHT.

8

HEAT RADIATION THROUGHBODY

through the stratum germinativum it is possiblethat some chemical action may be stimulated bythese rays in these very important lower skinlayers. In any case the hope of reaching deeptissue with these rays is rather a vain one.

From Figure 2 it is quite apparent that thescattering of the light increases greatly with thick-ness of the tissue. This explains the haziness ob-tained with infra-red photography as shown inFigure 6. Here are shown two photographs ofthe same surface, one (Plate I) made with theEastman panchromatic plate with no filter; theother (Plate II) made with the Eastman plateusing a filter. transmitted only infra-red radiationbetween 0.8 ,x and 1.5 ,u. The high reflectivity ofthe skin for visible light has completely maskedany subcutaneous detail in the first photograph,but in the second plate the lower reflecting powerfor the infra-red permits the photographing of theperipheral veins. The haziness is entirely due toscattering so that very fine detail could never beexpected of structures as far under the skin as%mm., also due to the great absorption of thetissue for the infra-red no pictures of structuresdeeper than 2 mm. can be expected. It mightbe pointed out that all of the detail in Plate II canbe easily seen with the naked eye.

SUMMARY

Evidence has been presented in the foregoingthat infra-red transmission through skin, even ofthe most penetrating rays, is of small proportion,about 95 per cent of these being absorbed within2 mm. of the surface and 99 per cent within 3 mm.of the surface. The use of dead skin in experi-ments on infra-red transmission has been shownto lead to too high rather than too low transmis-sion values so that the disagreement of the abovefindings with those of other investigators cannotbe ascribed to this cause. A method has been de-vised for the accurate measurement of scatteredtransmitted radiation. The absorption bands ofthe infra-red spectrum of skin have been identified.

Webelieve that the evidence justifies the fol-lowing conclusions:

1. The heating effect of infra-red radiation is

exerted principally on the body surface, and what-ever therapeutic effect is obtained by such radia-tion is due to this local effect and not to deeppenetration of the rays which is negligible inamount.

2. Photography of the human body in infra-red light, while yielding detail not obtainable byordinary photography, will not yield pictures ofstructures more than a few millimeters under thesurface and, due to scattering, will not give sharpdetail.

3. The absorption spectrum of normally wetskin is essentially that of liquid water. Upondrying, other absorption bands not due to waterbecome evident.

BIBLIOGRAPHY1. Hardy, J. D., and Muschenheim, C., The radiation of

heat from the human body. IV. The emission, re-flection and transmission of infra-red radiation bythe human skin. J. Clin. Invest., 1934, 13, 817.

2. Aldrich, L. B., Supplementary notes on body radia-tion. Smithsonian Misc. Collection, 1932, 85, No.11.

3. Cobet, R., and Bramigk, F., Uber Messung derWarmestrahlung der menschlichen Haut und ihreklinische Bedeutung. Deutsches Arch. f. klin.Med., 1924, 144, 45.

4. Hardy, J. D., The radiation of heat from the humanbody. III. The human skin as a black-body radia-tor. J. Clin. Invest., 1934, 13, 615.

5. Sonne, C., The mode of action of the universal lightbath. Acta med. Scandinav., 1921, 54, 336.

6. Loewy, A., and Dorno, C., Uber Haut- und Korper-temperaturen und ihre Beeinflussung durch physi-kalische Reize. Strahlentherapie, 1925, 20, 411.

7. Danforth, R. S., The penetration of living tissue byordinary radiant energy. Proc. Soc. Exper. Biol.and Med., 1930, 27, 283.

8. Cartwright, C. H., Infra-red transmission of theflesh. J. Optic. Soc. America, 1930, 20, 81.

9. Pauli, W. E., and Ivancevic, I., Untersuchungen uberdas Absorptionsvermogen der Haut im langwelligenGebiet des Spektrums. Strahlentherapie, 1927, 25,532.

10. Gaertner, O., Die Durchlissigkeit der menslichenHaut im Gebiete von 0.3-2.0 mu. Strahlentherapie,1931, 40, 377.

11. Christiansen, S., and Larsen, T., On the heat radiat-ing capacity of the human skin. Skandinav. Arch.f. Physiol., 1935, 72, 11.

12. Hutchins, C. C., Irregular reflection. Am. J. Sci.,1898, 6, 373.

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