I presume you want to know the wavelength at which to analyze a solution digested for COD.  For normal COD the wavelength is measured at 420 nm.  If you are using the special vials designed for low-level COD you want to use 600 nm.  In general, you need to adhere to whatever test method you are required to follow to figure out which wavelength to use.  If you are in the US, this link http://water.epa.gov/scitech/methods/cwa/methods_index.cfm will give you some good guidelines to follow.  Also if you can get a copy of Standard Methods for the Examination of Water and Wastewater, it will help you out.

I don't know how your particular spectrometer works, but you need to follow the instructions to set the wavelength.  For all traditional COD vials that have a range of 0 to around 1500 mg/L COD you will use 420 nm as your wavelength.  These vials have a dark orange-brown coloration and change to green with higher COD values.  While the range is listed as 0-1500 your lower detection limit is 50 or 100 depending on how well you make your standards.  Low range COD vials typically are designed for use from 0 to around 150 mg/L COD and are read at 600 nm.  These are typically a pale yellow color and become light blue to almost clear as the COD increases.  I still don't know if I have answered the question you are asking.  I blame language difficulties.  

If you want an official source showing 600 nm being the appropriate wavelength you will need to find a written method that is accepted by whoever is requiring you to do COD.  If you look at the link I provided in my first response, it will give a list of methods currently supported by the EPA.  Find 410.4, Determination of Chemical Oxygen Demand by Semi-Automated Colorimetry.  Section 2 says to read the digested samples at 600 nm.  That is the only free access I know of to a method.  If you have access to a copy of Standard Methods for the Examination of Water and Wastewater, consult 5220D.  I'm sure ASTM and USGS also have methods and there is probably a European version of the EPA that has an approved method.  Or you can simply take my word for it.