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.