296 THE INDIAN MEDICAL GAZETTE [May, 1942

MEASUREMENT OF RADIANT ENERGY

IN LIGHT THERAPY

By P. M. MEHTA, m.d., m.s., f.c.p.s. ? Director, Solarium, Jamnagar

Necessity of measurements Measurements are necessary to the progress

of any science or art; its understanding and advance greatly depend on the refinement and

organization of its measuring agents. Simplicity of measuring devices is one of the important factors which make a science adaptable to prac- tical use. In ray therapy the question of measurements presents one of its main difficulties.

This question is very much simplified in the case of chemical medications used in the current

system of medicine. The drugs are visible and can be easily measured by the measure-glass and the balance, fairly accurately. The im-

portant part the measure-glass and balance

play in medical practice is lost sight of, on

account of their simple and common nature.

But once 'take them away from the doctor's

chest, then will their indispensability and im-

portance become obvious. In ray therapy this question still remains

difficult and complicated, as it has to deal mostly with invisible agents which are not accurately known, whose production in uniform quality is not yet achieved and which undergo a number of changes during transmission.

Neither the sun nor the artificial sources of

light can be made to produce the required radia- tions in isolation. They produce them along with many other kinds of radiations. So we

have to measure not only the quantity but also determine the quality of radiations emitted

by them. Radiations undergo so many changes during transmission that it becomes necessary to measure them at the site of absorption instead of at the site of emission to ensure accuracy. Accurate measurement of such variable and

indefinite agents is impossible to achieve until we know them perfectly, and until we have

handy unerring instruments to measure them. We .cannot, however, wait till then. We must make our way with whatever implements we have. The highly technical instruments in use in

research laboratories which are capable of taking very minute measurements are unsuitable for

ray clinics. But there are simpler instruments which though not so fool-proof as the clinical thermometer can still be recommended, for fairly accurate and reliable results. We will describe the various instruments used for measurements in ray therapy.

Qualitative measurement

Spectroscope and spectrograph.?The spectro- scope and spectrograph provide perfect instru- ments for accurate qualitative determination.

By their means their wave-lengths as well as their intensities can be determined. The wave-

lengths are measured in metric units either of

Angstrom or micron. The spectrograph is the most essential instrument for the study of

radiations. In the spectroscope (figure 1) there is an

arrangement to receive the light for detection.

It is concentrated by means of a lens on the

prism where it is dispersed into its components. These are again focused on a fluorescent screen by a focusing or telescopic lens inserted between the prism and the screen. If the spectrum is to be photographed the camera should be inserted in place of the screen. This is a spectrograph. Another device used in the spectroscope is the

diffraction gratings ruled on a reflecting surface of speculum metal. Gratings ruled on a concave spherical surface will give a clear spectrum. This type of spectroscope is specially useful in the study of shorter wave-lengths (under 1,500A). The spectroscope measures the quality of

radiations present and the intensity of each wave-length. In knowing the value of any source of light the photo of spectral lines is not the complete guide, but it should be accompanied by the table or chart showing the relative inten- sity of each wave-length.

Source "j

Fig. 1.?Principle of spectrographs. Fig. 1.?Principle of spectrographs.

May, 19421 MEASUREMENT OF RADIANT ENERGY : MEHTA 297

Quantitative measurement

There is no direct method for quantitative measurement of radiations, so indirect methods have to be devised. The radiant energy is con- verted into other forms of energies which are measurable, or is made to cause chemical or

biological reactions which can be measured. The radiations to be measured are directed

on the receptor which is capable of absorbing them. Liberated radiant energy is transformed into an appreciable and measurable action. As conversion takes place into various other forms of energy, it will be useful to have a table of equivalents of various energy units here.

Table of units

Name of the unit 1 Definition Equivalent in other units

Angstrom

Erg

Watt

Micro-watt

Calorie

Calorie gm. minute ..

Milli. calorie gm. minute

Finsen

Abion

Unit of wave-length 1 1

I m/i = fj.~- 10 10,000 10.000,000

Unit for work 1

-kilogramme meter. 98,100,000

Unit of electromotive force or the force sufii- 10,000,000 ergs. cient to cause a current of 1 ampere to flow i

against a resistance of 1 ohm. 1

A millionth division of a watt j watts =10 ergs 1,000,000

Unit of heat therapy. The amount of energy j which could raise the temperature of 1 kilo- gramme of water from 0? to 1?C.

Same heat given in one minute 697,000 ergs. Unit for the measure of the ABC divisions of 697 ergs or 69 micro-watts, the U-V rays.

Unit for the measurement of U-V rays provoking erythema equivalent to the ery- themal effect on 1 cm.2 of 6,000 ergs trans- ported by radiation with wave-length of 2,967A.

Bactericide power of U-V rays killing 1 million staphylococci contained in 1 cm.3 of water with 7 per cent sodium chloride and exposed for 100 seconds at 10 cm. distance.

Ergs, watts, and gm. calories are the units of measurement of energy.

If the receptor is a chemical substance, the

change or colour or formation of precipitate would enable us to estimate the intensity of radiations.

In biological methods, cultures of microbes or protozoa are exposed to the radiation. The kill-

ing effects on the microbes and spores are com- pared to the fixed standard. For erythema human skin is exposed to ultra-violet radiation. These biological reactions depend upon in- dividual factors.

The following methods based on physical, chemical or biological .reactions are in common use for the measurement of the intensity of radiations possessing those properties :?

I. Biological reactions

(a) Erythema reaction on human skin (wave- length 2,400?3,100A).?1. The Council of

Physical Therapy in America has adopted this

reaction as the measure of efficiency of ultra- violet generations.

2. Erythema reaction is adopted as the main guide to ascertain the sensitiveness of the

patient's skin to decide the dose.

(b) Lethal action.?1. Killing of protozoa. Infusoria killing unit is fixed. The intensity of rays is measured by the time they take to kill the standard unit of infusoria culture.

2. Germicidal power.?In this standard cul- ture of bacteria is used (Jausion).

This method is made use of in commercial ultra-violet sterilization.

II. Fluorescence and phosphorescence These properties of certain wave-lengths are

made use of to detect their presence and some- times to measure their intensity.

III. Thermal effects

When any radiation is absorbed in a body, the radiant energy is converted into thermal

energy in the body. If there were a perfectly black body, it would completely absorb all wave- lengths of radiations. Consequent rises in tem- perature of the body would give us accurately the measure of total energy of radiations. Such a perfectly black body is not yet found, but a surface coated with lamp-black or platinum- black answers the requirement for all practical purposes. The construction of the principal measuring

apparatuses, viz, bolometer, thermopile and

micro-radiometer, is based on the transformation of radiant energy into heat. The simplest means

298 THE INDIAN MEDICAL GAZETTE [May, 1942

for measurement of heat is the mercury thermo- meter. But, as mercurial changes in thermo- meter are affected by a number of other factors, the thermometer cannot be regarded as a very accurate means of measurement in case of radiant energy. The thermocouple makes use of Seeback's dis-

covery that, when two conductors with varying temperatures are joined to form a circuit, a

direct continuous electric current flows in the cir- cuit as long as the difference in temperature is maintained. In it two metals with varying conductivity are used to make a junction so that the same heat produces unequal temperature in them. The thermocouple is covered with lamp- black and platinum-black which absorb almost all radiations and convert them into'heat energy. It is connected with a delicate galvanometer which registers the electric current which is in

proportion to the heat produced in the metal

pieces on absorption of radiations falling on

them. A thermopile consists of a number of thermo-

couples connected in series. The multiple junc- tions of thermo-elements make it an instrument of considerable accuracy for radiometric work.

By careful construction its sensitivity may equal that of the bolometer and it has the advantage of being more handy than the bolometer. These instruments do not deteriorate or go out of order

easily and hence they are mostly used in radio- therapy. The bolometer measures this heat by the in-

creased electrical resistance of strips of metal coated with platinum-black which are placed in an electric circuit. The resistance is measured

by a galvanometer attached to the instrument. By a bolometer it is possible to measure differ- ence in temperature so minute as one millionth of a degree. It is a very delicate instrument and therefore not of practical use in actinology.

In micro-radiometer the thermopile and gal- vanometer are combined in a single instrument. The instrument furnishes a very delicate means of measuring radiant energy.

Photo-chemical actions

Photo-chemical methods used to determine the

intensity of radiations are of two types:? 1. Photographic method. 2. Other photo-chemical methods. It is known that light causes many kinds of

chemical reactions, such as oxidation, reduction, etc. Light can ̂ accelerate certain chemical re-

actions, and, owing to its selective action, light can cause many photo-chemical reactions in

quite a specific manner. Many types of actinometers are contrived on

the basis of these properties of light to measure its intensity. Photography is the most

developed method. Changes in colour or transparency are the

means of measurements in other methods. The results of these chemical methods are dependent upon a number of factors such as temperature,

pressure, concentration, purity of chemicals, etc. They cannot be considered so accurate as

thermal or electric methods of measurement.

Photo-electric effects Light is converted into electric energy under

certain circumstances. Every substance has a

critical or threshold wave-length, that is, when radiations of shorter wave-lengths than thres- hold wave-length fall upon it, it begins to

emit electrons. Wave-lengths below 3,000A generally produce these effects in most sub- stances. There are some metals where these effects are produced by longer wave-lengths. The number of electrons given off is in direct

proportion to the intensity of the radiations. This property is utilized to detect the presence and measure the intensity of ultra-violet radia- tion specially.

For practical purposes, only a few metals exhibit any appreciable useful sensitivity to

light, and, since all tarnish in air, it is necessary to enclose them in a vacuum or inert gas in a

glass cell. The current may be measured by ordinary instruments, such as the electrometer or electroscope or voltmeter; the current may be amplified before measurement. The instru- ments used are the photometer or photo-electric cell. These instruments are very delicate and deteriorate rapidly. Photo conductivity.?In some substances the

effect of light is to increase their conductivity. No new electric energy is produced but there is easier conduction of the same current. This

property is specially marked in selenium. Selenium photo-electric cells are therefore used to measure the intensity of ultra-violet rays.

Instruments used in the solarium We give below the brief description of the

instruments we used in the solarium for the measurement of radiations?solar as well as

those from artificial sources. I. Helio-actinometer (figure 2).?The helio-

actinometer for the measurement of solar radia- tions consists of a thermopile connected with a galvanometer. The thermopile consists of ten

or nineteen junctions. The thin plates which are made of an alloy of nickel and alloy of constantan are 5 millimetres broad and 30 milli- metres long. The extremities of these plates are fixed to a brass block or to the tubes forming radiators for dispensing the heat absorbed.

There is a groove in the brass block or between the tubes to receive the thermometer to measure the temperature. The latest model of 1940 has been improved so as to show the value of each junction separately also. The area of thermopile exposed to the sun

is 7 cm. by 2 cm., or 12 cm. by 2 cm. Each

junction can carry one millivolt of current. The galvanometer is also specially modified.

Tnere is direct movement of the needle and lowest possible resistance is arranged. The dial on which the needle moves shows one hundred

May, 1942] i MEASUREMENT OF RADIANT ENERGY : MEHTA 299

divisions the total value of which is approxi- mately 1.5 gramme-calories.

There are four such thermopiles used for the measurement of solar radiations:?

1. For measuring the intensity of total radiations.

2. For measuring the intensity of infra-red radiation. This thermopile is covered with a

glass of selenium oxide which allows only the red and infra-red rays to fall on thermopile.

3. The pyrheliometer. The thermopile is

put in a cylinder ten inches high to prevent the diffused radiation from falling on the thermopile.

4. For measuring the intensity of the ultra- violet radiations, there is no filter which allows

only ultra violet to pass. Even the best ultra- violet filters are transparent to some infra-red rays. So accurate measurement of ultra violet is not possible. To obviate this difficulty a

contrivance is made by connecting two thermo- piles in opposite directions. The first thermo- pile is covered with ultra-violet filter and an

infra-red filter under it; and-the second one is covered with ultra-violet filter alone. The first

thermopile receives only infra-red rays while the second receives infra-red and ultra-violet rays. The current produced by infra red in both will be neutralized leaving the current produced by ultra violet to be measured by the galvanometer. To avoid frequent calculation of the time of

exposure due to the variations in the intensity

of solar radiations and thus to facilitate the accurate dosage, a special counter-helio-actino- meter or totalizator (figure 3) is designed

whereby the exact intensity of solar radiation is automatically registered in solar units. This

apparatus consists of a thermopile of 19 or

50 junctions to receive sufficient amount of solar energy to move a small rotator raised on a pivot and placed between the extremities of a powerful magnet. This works on the principle of electric motor. The revolutions of the motor are

recorded on a dial with unit marks. The needle moves slowly or briskly according to the motion of the motor which depends on the intensity of radiation. One unit is equivalent to 2.5 gramme- calories.

II. Spectrograph.?There is a specially prepared spectrograph for continuously photographing the spectrum of ultra-violet constituent of sunlight. The sunlight and diffused skylight are reflected

on the slit of the spectrograph from a white plate of pure magnesium. This light is concentrated on the slit by means of a quartz lens and nickel oxide filter is inserted to remove visible radia- tions which would otherwise eclipse the spectrum of ultra violet. The spectrum is being con-

tinuously photographed from sunrise to sunset on a standard 35 mm., film roll turning on a rotating cylinder.

o

Fig. 2.?Helio-actinometer. Fig. 2.?Helio-actinometer.

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Fig. 3.?Counter-helio-actinometer. Fig. 3.?Counter-helio-actinometer.

300 THE INDIAN MEDICAL GAZETTE [May, 1942

III. Photo-electric cell (figure 4).?Silver selenium alloy is used to produce the photo- electric effects which are shown on a very delicate galvanometer. As visible and ultra violet both produce photo-electric effects on

silver selenium, a nickel oxide filter is used to cut off the visible rays.

IV. Chemical actinometers.?Two types of chemical actinometers are used in the solarium. One is meant to ascertain existence and amount of erythema producing wave-lengths in the emission of a particular source of light. A filter- paper soaked in a colourless solution of para- phenylendiamine is exposed to the light. The

filter-paper will become greyish green and then dark grey in proportion to the quantity of erythe- ma producing wave-lengths in the light. A disc of standard colour is kept for comparison. Another actinometer is called the leucobase.

This consists of a short quartz tube which con- tains an alcoholic solution of leuco-cyanure tri- phenylmethane which is normally colourless. The reagent takes various grades of pink colour when exposed to ultra violet shorter than 3,150A according to the intensity of radiations. A colorimeter containing ten graded discs of colour is attached to the instrument of fixing the value of colour change in the quartz tube.

V. Sunshine recorder (figure 5).?It is an

instrument to record the intensity of bright

sunshine. Sun's rays are focused by means of a glass sphere upon specially prepared and printed cardboard strips. The frame carrying the chart must be tilted and clamped to the correct lati- tude of the place of observation. Various grades of charring occur on the strip according to the intensity of sunshine. VI. Konimeter (figure 6).?Konimeter is

an instrument for counting the number of dust

particles in the atmosphere. A measured quan- tity of air is sucked by a pump through a jet passed through a circular glass plate mounted to rotate. The fine jet of air impinges perpendi- cularly upon the moistened plate and in so doing deposits there its dust contents. The arc of

glass plate on which dust is deposited is put under the microscope and the number of dust

particles counted. As dust particles play an

important part in the diffusion of solar ultra- violet radiation, the counting of dust particles in the atmosphere is a useful procedure in a

helio-observatory.

T?V/?NSPARBNr J 111

| J 1

Fig. 4.?Photo-electric cell, with galvanometer. Fig. 4.?Photo-electric cell, with galvanometer.

Fig. 5.?Sunshine recorder. Fig. 5.?Sunshine recorder.

Fig. 6.?Konimeter. Fig. 6.?Konimeter.