  One of the problems of producing instruments for Extremely Large Telescopes
is that their size (and hence cost) scales rapidly with telescope aperture. To
try to break this relation alternative new technologies have been proposed,
such as the use of the Integrated Photonic Spectrograph (IPS). Due to their
diffraction-limited nature the IPS is claimed to defeat the harsh scaling law
applying to conventional instruments. In contrast to photonic applications,
devices for astronomy are not usually used at the diffraction limit. Therefore
to retain throughput and spatial information, the IPS requires a photonic
lantern (PL) to decompose the input multimode light into single modes. This is
then fed into either numerous Arrayed Waveguide Gratings (AWGs) or a
conventional spectrograph. We investigate the potential advantage of using an
IPS instead of conventional monolithic optics for a variety of capabilities
represented by existing instruments and others planned for Extremely Large
Telescopes. We show that a full IPS instrument is equivalent to an
image-slicer. However, the requirement to decompose the input light into
individual modes imposes a redundancy in terms of the numbers of components and
detector pixels in many cases which acts to cancel out the advantage of the
small size of the photonic components. However, there are specific applications
where an IPS gives a potential advantage which we describe. Furthermore, the
IPS approach has the potential advantage of minimising or eliminating bulk
optics. We show that AWGs fed with multiple single-mode inputs from a PL
require relatively bulky auxiliary optics and a 2-D detector array. A more
attractive option is to combine the outputs of many AWGs so that a 1-D detector
can be used to greatly reduce the number of detector pixels required and
provide efficient adaptation to the curved output focal surface.
