A transparent display apparatus, comprising: a transparent display unit (110, 210) having light emitting pixel elements and transmitting at least a portion of first light, the first light being incident on the back of the transparent display apparatus; at least one sensor (150, 610, 716) configured to generate at least one signal indicative of brightness of the first light; a light control unit (120, 230, 300) overlapping with the transparent display unit (110, 210) and adapted to transmit a portion of the first light to the transparent display unit (110, 210), wherein the transparent display apparatus is adapted to control the light control unit (120, 230, 300) by applying a voltage having a duration and an intensity, the light control unit (120, 230, 300) having an adjustable transmittance having at least a minimum transmittance, a maximum transmittance, and a transmittance between the minimum transmittance and the maximum transmittance for the first light, the transmittances of the light control unit (120, 230, 300) being dependent on the applied voltage, characterised in that: the transparent display apparatus further comprises a gamma voltage generating unit configured to convert a digital input video into an analog voltage, to apply the converted voltage as a gamma voltage to the transparent display unit, and to adjust a maximum luminance by controlling a maximum value of the gamma voltage corresponding to the video input, and the transparent display apparatus is adapted to calculate a contrast ratio by dividing a sum of the maximum luminance plus a luminance of the first light transmitted through the transparent display apparatus by a sum of a minimum luminance plus the luminance of the first light transmitted through the transparent display apparatus, and wherein an amount of the portion of the first light transmitted through the transparent display unit (110, 210) is adjusted by adjusting the adjustable transmittance of the light control unit (120, 230, 300) according to the applied voltage based on the signal indicative of the brightness of the first light and based on a target contrast ratio of the transparent display apparatus such that the contrast ratio of the transparent display apparatus is maintained to be approximate to the target contrast ratio. a transparent display unit (110, 210) having light emitting pixel elements and transmitting at least a portion of first light, the first light being incident on the back of the transparent display apparatus; at least one sensor (150, 610, 716) configured to generate at least one signal indicative of brightness of the first light; a light control unit (120, 230, 300) overlapping with the transparent display unit (110, 210) and adapted to transmit a portion of the first light to the transparent display unit (110, 210), wherein the transparent display apparatus is adapted to control the light control unit (120, 230, 300) by applying a voltage having a duration and an intensity, the light control unit (120, 230, 300) having an adjustable transmittance having at least a minimum transmittance, a maximum transmittance, and a transmittance between the minimum transmittance and the maximum transmittance for the first light, the transmittances of the light control unit (120, 230, 300) being dependent on the applied voltage, the transparent display apparatus further comprises a gamma voltage generating unit configured to convert a digital input video into an analog voltage, to apply the converted voltage as a gamma voltage to the transparent display unit, and to adjust a maximum luminance by controlling a maximum value of the gamma voltage corresponding to the video input, and the transparent display apparatus is adapted to calculate a contrast ratio by dividing a sum of the maximum luminance plus a luminance of the first light transmitted through the transparent display apparatus by a sum of a minimum luminance plus the luminance of the first light transmitted through the transparent display apparatus, and wherein an amount of the portion of the first light transmitted through the transparent display unit (110, 210) is adjusted by adjusting the adjustable transmittance of the light control unit (120, 230, 300) according to the applied voltage based on the signal indicative of the brightness of the first light and based on a target contrast ratio of the transparent display apparatus such that the contrast ratio of the transparent display apparatus is maintained to be approximate to the target contrast ratio.