Response to Hello Joanne.  It appears from the photo provided that the fixture is a Pentair Pool Products Light Fixture.  Please see the attached link to a Pentair Pool Products Light Fixture.  Amerlite 500W 120V 50' Pool Light - EC-602128   Please compare the design on the stainless steel light ring to the fixture installed in your pool for a match.  Seeking confirmation of the light manufacturer which will determine the correct lens gasket replacement.  Your fixture may have the manufacturer's label still attached.  The lens gasket 79101600-0-170 is the correct gasket for a Pentair Light Fixture.  Lens Gasket - 79101600 - O-170     How to Replace a Pool Light Bulb

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