, ▁and ▁could ▁be ▁focused ▁on ▁a ▁specific ▁region ▁of ▁the ▁spectrum ▁by ▁til ting ▁the ▁gr ating . ▁ ▁The ▁limitation ▁to ▁a ▁bla zed ▁gr ating ▁is ▁the ▁width ▁of ▁the ▁mirror s , ▁which ▁can ▁only ▁be ▁ground ▁a ▁finite ▁amount ▁before ▁focus ▁is ▁lost ; ▁the ▁maximum ▁is ▁around ▁ 1 0 0 0 ▁lines / mm . ▁In ▁order ▁to ▁overcome ▁this ▁limitation ▁h olog raph ic ▁grat ings ▁were ▁developed . ▁ ▁Volume ▁phase ▁h olog raph ic ▁grat ings ▁use ▁a ▁thin ▁film ▁of ▁dich rom ated ▁gel atin ▁on ▁a ▁glass ▁surface , ▁which ▁is ▁subsequently ▁exposed ▁to ▁a ▁wave ▁pattern ▁created ▁by ▁an ▁inter fer ometer . ▁This ▁wave ▁pattern ▁sets ▁up ▁a ▁reflection ▁pattern ▁similar ▁to ▁the ▁bla zed ▁grat ings ▁but ▁util izing ▁B rag g ▁diff raction , ▁a ▁process ▁where ▁the ▁angle ▁of ▁reflection ▁is ▁dependent ▁on ▁the ▁arrangement ▁of ▁the ▁atoms ▁in ▁the ▁gel atin . ▁The ▁h olog raph ic ▁grat ings ▁can ▁have ▁up ▁to ▁ 6 0 0 0 ▁lines / mm ▁and ▁can ▁be ▁up ▁to ▁twice ▁as ▁efficient ▁in ▁collect ing ▁light ▁as ▁bla zed ▁grat ings . ▁Because ▁they ▁are ▁se aled ▁between ▁two ▁sheets ▁of ▁glass , ▁the ▁h olog raph ic ▁grat ings ▁are ▁very ▁vers atile , ▁potentially ▁last ing ▁dec ades ▁before ▁need ing ▁replacement . ▁ ▁Light ▁dispers ed ▁by ▁the ▁gr ating ▁or ▁pr ism ▁in ▁a ▁spect ro graph ▁can ▁be ▁recorded ▁by ▁a ▁det ector