亮度計主(zhu)要(yao)(yao)采(cai)用(yong)一對有一定(ding)(ding)(ding)距離的(de)光(guang)孔(kong)(kong)接(jie)收固(gu)定(ding)(ding)(ding)立體(ti)角、固(gu)定(ding)(ding)(ding)投光(guang)面(mian)積(ji)的(de)光(guang)通量(liang)，此值不隨(sui)物體(ti)遠(yuan)近而(er)變，只要(yao)(yao)物體(ti)的(de)表(biao)面(mian)積(ji)足(zu)夠大。為了瞄準(zhun)被(bei)測物體(ti),常采(cai)用(yong)成(cheng)像(xiang)系統。被(bei)測光(guang)源經物鏡(jing)(jing)后在帶(dai)孔(kong)(kong)（前光(guang)孔(kong)(kong)）反(fan)(fan)射(she)鏡(jing)(jing)上成(cheng)像(xiang)，其中一部分經反(fan)(fan)射(she)鏡(jing)(jing)及目(mu)鏡(jing)(jing)，由人(ren)眼接(jie)收，以(yi)瞄準(zhun)和(he)監控(kong)清晰成(cheng)像(xiang)面(mian)與帶(dai)孔(kong)(kong)反(fan)(fan)射(she)鏡(jing)(jing)重(zhong)合；另一部分光(guang)則(ze)經過反(fan)(fan)射(she)鏡(jing)(jing)上的(de)小孔(kong)(kong)經后光(guang)孔(kong)(kong)到達V(λ)接(jie)收器。亮度值用(yong)指針或數字表(biao)頭顯示。