A method for detecting a test gas cloud escaping from a leak in a test specimen , comprising the following steps: receiving optical radiation reflected or emitted from the test specimen or its background at a first point in time with an optical sensor configured to detect at least one wavelength or a wavelength range of the optical absorption spectrum of the test gas , creating a first digital image from the optical radiation received at the first point in time such that the signal amplitudes of the image points x ij correspond to the amplitude of at least one absorption wavelength range of the test gas , receiving optical radiation reflected or emitted from the test specimen or its background at the second point in time by means of the optical sensor , creating a second digital image from the optical radiation received at the second point in time such that the signal amplitudes of the image points x ij correspond to the amplitude of at least one absorption wavelength range of the test gas , comparing the first image with at least one second digital image of the reflected optical radiation, which is different from the first image, wherein a leak is considered detected when at least the difference of the amplitude of at least a first image point x ij of the first image and the amplitude of at least a second image point x ij of the second image exceeds a threshold value, wherein i, j are each natural numbers x ij is an image point at the location of column i and row j of the first image , and x ij is an image point at the location of column i and row j of the second image characterized in that for integral evaluation of the tightness of the test specimen , the sum is formed from the differences of the amplitudes of mutually corresponding image points x ij of the first image and x ij of the second image , a leak being regarded as detected if the sum exceeds the threshold value. receiving optical radiation reflected or emitted from the test specimen or its background at a first point in time with an optical sensor configured to detect at least one wavelength or a wavelength range of the optical absorption spectrum of the test gas , creating a first digital image from the optical radiation received at the first point in time such that the signal amplitudes of the image points x ij correspond to the amplitude of at least one absorption wavelength range of the test gas , receiving optical radiation reflected or emitted from the test specimen or its background at the second point in time by means of the optical sensor , creating a second digital image from the optical radiation received at the second point in time such that the signal amplitudes of the image points x ij correspond to the amplitude of at least one absorption wavelength range of the test gas , comparing the first image with at least one second digital image of the reflected optical radiation, which is different from the first image, wherein a leak is considered detected when at least the difference of the amplitude of at least a first image point x ij of the first image and the amplitude of at least a second image point x ij of the second image exceeds a threshold value, wherein i, j are each natural numbers x ij is an image point at the location of column i and row j of the first image , and x ij is an image point at the location of column i and row j of the second image i, j are each natural numbers x ij is an image point at the location of column i and row j of the first image , and x ij is an image point at the location of column i and row j of the second image characterized in that for integral evaluation of the tightness of the test specimen , the sum is formed from the differences of the amplitudes of mutually corresponding image points x ij of the first image and x ij of the second image , a leak being regarded as detected if the sum exceeds the threshold value. <EOS>