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With the aim of extracting some very basic and important parameters of compact H II regions, (viz., the type of the embedded star; the density and the temperature distribution in the cloud; the radial optical depth etc.), they have been modelled realistically and radiation transfer calculations were performed to predict their emergent SEDs, angular sizes etc. By exploring a realistic parameter space of several variables, it has been possible to identify those parts of the continuum SED, which play more significant role in deriving physical details of the compact H II regions. The model predictions when compared with observations, would quickly lead to conclusions regarding those parameters. This study is particularly a first step towards more detailed modelling of any target source by fine tuning the parameters further or introducing more details into the model. A more generalized study with fewer constraints and modelling of fine structure line emission from heavy elements in gas phase are presented in Mookerjea & Ghosh (1999, Paper II).

In order to have a conservative estimate of the colours, only those parts of the spectra are defined to be ``measurable'', where the emission from the cloud is substantially more than the contribution from the ISRF scattered by the outer regions of the cloud. As a result, the near infrared (< 3 µm) spectra are entirely unusable and between 5 & 10 µm, the spectral details are useful depending on the exciting star (O4 or O7 or B0.5).