Umbono we-20/20 ungumlinganiso wezinto ezibonakalayo. Ubuciko obubonakalayo buka-20/20 busho ukuthi umuntu angabona imininingwane encane kusuka kumamitha angu-20 kuyafana nomuntu onamehlo ajwayelekile angabona kusuka kumamitha angu-20. Uma umuntu enobuciko obubonakalayo obuyi-20/40, kuthiwa ubona imininingwane esuka kumamitha angu-20 kuyafana nomuntu onamehlo ajwayelekile angayibona kusuka kumamitha angu-40 kude.
U-ophthalmologist waseDutch, uHerman Snellen wakhetha u- Snellen ubuciko obubonakalayo ngo-1862. Uhlamvu lwe-Snellen olubonakalayo lubalwa njengengxenyana futhi luboniswa njengendlela ibanga elihlolwa ngalo ibanga lapho i-optotype encane ekhonjisiwe ibonakala i-angle yemizuzu emihlanu ye-arc imininingwane yalowo mbhalo uhlobisa iminithi engu-1 ye-arc. I-optotype iyincwadi eqinisekisiwe, ukuma, isithombe noma inombolo esetshenziselwa ukukhombisa isihloko esibukeka ngayo ubuningi bokubukeka.
Umbono we-20/20 ubhekwa njengokujwayelekile kodwa akusho ukuthi umbono ophelele. Ukuba nombono we-20/20 ubonisa ukucaca kombono ekude. Akusho lutho mayelana nekhwalithi yombono namakhono okudingeka abe nombono omuhle njengokubambisana kwamehlo, ikhono lokuhlala, ukuqonda okujulile, umbono wombono, noma umbono wombala. Ngaphezu kwalokho, ukuba nombono we-20/20 akusho ukuthi iso lakho liphilile. Umuntu angaba ne- glaucoma enzima kodwa namanje agcine umbono we-20/20 ophakathi.
Futhi, umuntu angabheka kude , kodwa namanje ulinganisa ubuciko obubonakalayo obungama-20/20 ngaphandle kwezibuko. Abantu ababheke phambili bezinye izikhathi banalo ikhono lokugxila kakhudlwana futhi balinganisa ukubhekwa phambili. Kodwa-ke, ukubambelela lapho isikhathi eside kududuza futhi ekugcineni, umbono ubonakala uphazamisekile.
Umbono we-20/20 ubhekwa njengombono ovamile.
Kungenzeka ukuba nombono ophakeme kunama-20/20: ubuhle obukhulu beso lomuntu ngaphandle kwezinsiza ezibukwayo (njengama-binoculars) ngokuvamile kucatshangwa ukuthi kuphakathi kuka-20/10 noma ngabe abantu abambalwa babhalisiwe benombono we-20/5.
Ezinye Izindlela Zokulinganisa Ukubukeka Okubonakalayo
"Ukubamba u-E" Ishadi Lokubukeka Lokubonakalayo - Ulinganisa kanjani ubuciko obubonakalayo kumuntu ongakwazi ukukhuluma noma mhlawumbe nje usamahloni ukukhuluma? Ishadi E-Tumbling E lisebenzisa isilinganiso esilingana nesilinganiso njengeStellen Eye Ishadi Lamazwi. Kodwa-ke, wonke ama-optotypes asetshenziswe eshadini yi-capital E kodwa akhonjiswe ngamasu ahlukene. Kule ndaba, umuntu oboniswa incwadi okumele ayifunde eshadini lesamehlo angasebenzisa iminwe yakhe futhi abonise umhloli ukuthi iyiphi indlela E ekhomba ngayo.
I-LEA Vision Test - Isivivinyo se-LEA sivivinywa sezinto ezibonakalayo ezenzelwe abantwana abangakwazi ukufunda izincwadi. Ukuhlolwa kusetshenziselwa ama-optotype: i-apula, i-pentagon, isikwele kanye nombuthano. Ukuhlolwa kusebenzisa lezi zimpawu ezivamile ukuze ubuciko obubonakalayo bungalinganiselwe kuncane kakhulu.
IJeeger Ishadi Lamehlo Elikufuphi Lombono - Uma uke wahlolwa, ungase uboniswe ishadi leyeso leJaeger ukuze uvivinye ubuciko bakho obuseduze bokubukwa. Ishadi leJaeger ishadi elincane eliphathekayo elibonisa amabhlogo wombhalo ngobukhulu obuhlukahlukene.
Esikhundleni sokusebenzisa leli gama 2/20, ishadi le-Jeager lisebenzisa isilinganiso esingaba yi-J1 kuya ku-J11. I-J2 ivame ukubhekwa njengokulingana nokulinganisa okujwayelekile 20/20. Ishadi ngokuvamile lihamba phakathi kuka-12-16 amasentimitha ukusuka emehlweni akho.