Gwajin hangen nesa, gwajin Ishihara ya fi sha'awar fahimtar launuka. A yau shi ne gwajin da aka fi amfani da shi a duk duniya don gano nau'ikan makanta daban-daban.

An yi tunanin a cikin 1917 ta farfesa na Japan Shinobu Ishihara (1879-1963), gwajin Ishihara shine jarrabawar chromatic don tantance fahimtar launuka. Yana ba da damar gano wasu gazawar da ke da alaƙa da hangen nesa mai launi (dyschromatopsia) waɗanda aka haɗa a ƙarƙashin kalmar makanta launi.

Gwajin dai an yi shi ne da alluna 38, wanda aka yi shi da mosaic na dige-dige masu launi daban-daban, wanda a cikinsu siffa ko lamba ke bayyana albarkacin raka'a na launuka. Don haka ana gwada majiyyaci akan iya gane wannan siffa: makaho mai launi ba zai iya bambanta zanen ba saboda bai fahimci launinsa daidai ba. An raba gwajin zuwa jeri daban-daban, kowanne an tsara shi zuwa wani ƙayyadadden ƙayyadaddun yanayin.

Ana yin gwajin ne a ofishin likitan ido. Ya kamata majiyyaci ya sanya gilashin gyaran fuska idan yana bukatar su. Yawancin idanu biyu ana gwada su a lokaci guda.

Ana gabatar da faranti ɗaya bayan ɗaya ga majiyyaci, wanda dole ne ya nuna lamba ko nau'in da ya bambanta, ko kuma babu form ko lamba.

Ana yin gwajin Ishihara ne idan ana zargin makanta launi, misali a cikin iyalai masu launin launi (abin da ba a sani ba shi ne mafi yawan asalin kwayoyin halitta) ko lokacin jarrabawar yau da kullun, misali a kofar shiga makarantar.

Sakamakon gwajin yana taimakawa gano nau'ikan makanta daban-daban:

• protanopia (mutum ba ya ganin ja) ko protanomaly: an rage fahimtar ja
• deuteranopia (mutum bai ga kore ba) ko deuteranomaly (hankalin kore yana raguwa).

Kamar yadda gwajin yana da inganci kuma ba ƙididdigewa ba, ba zai yiwu a gano matakin harin mutum ba, don haka don bambanta deuteranopia daga deuteranomaly, alal misali. Ƙarin bincike mai zurfi na ophthalmologic zai sa ya yiwu a ƙayyade nau'in makanta launi.

Har ila yau, gwajin ba zai iya tantance tritanopia ba (mutumin ba ya ganin rauni da tritanomaly (raguwar fahimtar shuɗi), waɗanda ba su da yawa.

Babu magani a halin yanzu da zai yiwu don rage makanta launi, wanda haka ma ba ya haifar da nakasu na yau da kullun, kuma baya canza yanayin hangen nesa.