. Light and lighting. ich 4iS THE ILLUMINATING ENGINEER (sept.) may be 0*25 candles per square centi-metre. As illustrations of low brighlwe may take the illuminated walls in aroom in an ordinary lighting installationwhere 00005 candle-power pel squarecentimetre or less, is common. In measuring the brightness of suchobjects as incandescent filaments theplan is to determine the total candle-power and to divide tins by the measuredsuperficial area of the filament. Having thus obtained the brightness of a standard filament under specified electrical con- This leads us to the third method—direct o
. Light and lighting. ich 4iS THE ILLUMINATING ENGINEER (sept.) may be 0*25 candles per square centi-metre. As illustrations of low brighlwe may take the illuminated walls in aroom in an ordinary lighting installationwhere 00005 candle-power pel squarecentimetre or less, is common. In measuring the brightness of suchobjects as incandescent filaments theplan is to determine the total candle-power and to divide tins by the measuredsuperficial area of the filament. Having thus obtained the brightness of a standard filament under specified electrical con- This leads us to the third method—direct observation. During the recent-vears there has sprung up a new form ofphotometer adapted for measurementsof surface brightness. When such instruments are used tomeasure illumination the usual plan is toplace a standard matt white surface atthe place where the illumination is to bemeasured and to observe this from adistance. This standard surface isviewed through an aperture in thephotometric screen and the brightness. Fig i View of an artificially lighted interior showing brightness (in candles persquare centimetre) >>i various objects. (Dr. II 1. 1 ditions we can compare the brightness ofothers Ipv Buper-imposing them on itsimage and then varying the current of thestandard filament until both appealequally bright. In the case of a diffusing globe it is Usually most convenient to .screen oft all t;1jn small measured areaand to determine the candle-power of this element. A- a rule the brightness of Midia globe is tOO higfa to he measured directly. of the latter is adjusted to equality withit. This standard surface is always usedin calibrating the instrument for illumina-tion. When the instrument is used to measurebrightness of surrounding objects all thatis necessary is to observe them through theaperture iii just the same manner as thestandard screen. (It is important to ootid that the apparent brightness of anobject is the .same irrespective of the THE ILLUMINATING ENGI
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