Markus Degünther

Influence of inhomogeneous surface albedo on UV irradiance: Effect of a stratus cloud

The influence of inhomogeneous surface albedo on UV irradiance is investigated assuming the sky to be overcast by a horizontally homogeneous stratus cloud. From calculations with a three-dimensional (3-D) radiative transfer model the snow-induced relative UV irradiance enhancement as well as the 1-D albedo is derived for two different snow bidirectional reflectance distribution functions (Lambertian and anisotropic) and snow coverages varying between 0 and 100%. The 1-D albedo is a quantity defined to consider effects of inhomogeneous surface albedo even in 1-D radiative transfer codes. It turned out that anisotropy of snow reflectance is of minor importance for quantification of inhomogeneous surface albedo influence on UV irradiance. This holds for a cloudy as well as for a cloud-free atmosphere. Furthermore, the comparison of the new results with corresponding data obtained for cloudless sky indicates that a stratus cloud, on the one hand, enhances the surface albedo effect by about a factor 2–3, depending on wavelength. On the other hand, the area significantly influencing UV irradiance via its surface albedo is distinctly smaller. Looking, e.g., at wavelength 330 nm and clear sky, the maximum albedo effect of the surface surrounding a quadratic area of 6400 km2 on UV irradiance at area center is 3%. In the case of a stratus cloud the same effect is found for an area of only 1500 km2.

Performance of a programmable illuminator for generation of freeform sources on high NA immersion systems

This paper describes the principle and performance of a fully programmable illuminator for a high-NA immersion system. Sources can be generated on demand, by manipulating an array of mirrors instead of the traditional way of inserting optical elements and changing lens positions. All mirrors are always used to create the source such that no light is lost when switching from one source shape to another. Measured sources generated with this new type of illumination system will be shown and compared to the target sources generated by source mask optimization software or targets of traditional sources. Comparison between measured and target source will be done both in parameters of a pupil fit model and by simulated imaging impact. Also the first results in resist obtained on a XTIV 1950Hi 1.35 NA tool equipped with this illuminator are presented and compared to measurements on the same system when it was equipped with an Aerial XP illumination system.

A Radiative Transfer Case Study for 3‐d cloud effects in the UV

Satellite UV mapping is usually based on the independent pixel approximation (IPA) which neglects horizontal photon transport between adjacent columns. Horizontal inhomogeneity of cloud fields therefore causes uncertainties in the derived UV radiation fields. While these effects are small for large pixel satellites, the broken-cloud errors increase as the pixel size decreases. By comparing results of 1-d and 3-d UV radiative transfer calculations for three selected cloud scenes that cover a rather broad range of cloud inhomogeneity the main 3-d cloud effects on the atmospheric UV transmission are identified and quantified in their order of magnitude. With respect to the different spatial resolutions of satellite instruments it is further shown how 3-d cloud effects average out with increasing spatial scale. It turns out that locally the IPA cause maximum uncertainties up to ±100% for a spatial resolution of about 1 × 1 km² (e.g., AVHRR), they are reduced to ±10% for a resolution of about 15 × 15 km² and below 5% for a resolution greater than 30 km (e.g., TOMS).