@Dunge That's an excellent way to determine when a bulb is bad and is a great idea!!
I did a little looking around quick as it sparked my attention. I'll post some various aspects related to this in case it can help. I'll clip out some items related to the spectral shift and performance drop over time.
I'm sure most know how these operate, but to cover for clarity:
LINK (wiki)
"High-pressure sodium (HPS) lamps are smaller and contain elements such as mercury...." "The sodium D-line is the main source of light from the HPS lamp, and it is [URL='http://en.wikipedia.org/wiki/Spectral_line#Pressure_broadening']pressure broadened by the high pressure in the lamp". "Xenon at a low pressure is used as a "starter gas" in the HPS lamp."[/URL]
http://en.wikipedia.org/wiki/Mercury_(element)
"An amalgam of metallic sodium and mercury lies at the coolest part of the lamp and provides the sodium and mercury vapor that is needed to draw an arc." .... "The higher the temperature of the amalgam, the higher will be the mercury and sodium vapor pressures in the lamp and the higher will be the terminal voltage. As the temperature rises, the constant current and increasing voltage result in increased power until the nominal power is reached."
Further insight into design and operation characteristics from another source
LINK
For mercury and MH bulbs
"When a mercury vapor or metal halide lamp is energized, an electrical field is generated between one of the main electrodes and the starting electrode next to it. This causes an emission of electrons that ionize the argon starting gas. The ionized argon particles create a diffused argon arc between the two main electrodes of the lamp (Figure 4). The heat from this argon arc gradually vaporizes the arc metals in the arc tube. These ionized arc metal particles join the arc stream between the two main electrodes. When a sufficient number of ionized particles join the arc stream, the resistance between the main electrodes drops to a point where the start-up voltage supplied by the ballast can strike a current arc across the main electrodes. The arc current continues to increase until the current rating of the lamp is reached"
For HPS
"When an HPS lamp is energized, the high-voltage pulse ionizes the xenon gas in the arc tube, and an arc is established between the main electrodes. As soon as this arc is established, the voltage pulse is switched off. Sodium and mercury arc metals quickly vaporize and join this arc stream, and the arc current increases and stabilizes."
That same link has some good information in regards to spectrum and how its' achieved. This may be an aspect that is often not considered.
"The HID arc consists of a very rapid flow of both electrons and charged arc metal ions. During this rapid movement, countless collisions occur between ions and electrons. As these particles collide, they release energy at a specific wavelength (Figure 5). This energy appears to us as light. Because the number of particles in the arc tube is so great and the occurrence of collisions so frequent, it appears that the entire arc path constantly generates light."
Spectral production for MH and Mercury
"The color of the light is a characteristic of the light spectrum wavelength of the arc metals contained in the arc tube. For example, in a mercury vapor lamp, the mercury produces a distinct greenish white-blue light. Red, orange and yellow hues appear grayish. In a metal halide lamp, the arc discharges through the combined vapors of mercury and certain compounds of iodine. The halide compounds help strengthen yellows, greens and blues, so the overall color rendering of metal halide lamps is green-white. Reds and oranges appear dulled. Phosphor coatings on the bulbs of mercury vapor and metal halide lamps can improve color rendering and provide light diffusion."
Spectral production for HPS
HPS lamps generate a sodium-based light that is strongest in the yellow and orange range of the spectrum and weakest in the blue-green wavelengths. A small amount of mercury is added to the arc tube to help strengthen blues and greens, but the overall color rendering is still golden white, with both reds and blues appearing grayed.
So, now we know that high voltage, noble gasses and vaporized metals are producing the light. We also know that the spectrum of the light is based off both the metals contained and their proportion. Specifically for HPS we need to account for some other factors before looking at wear over the life.
Taken from the first link above (wiki)
"Because of the extremely high chemical activity of the high-pressure sodium arc, the arc tube is typically made of translucent aluminum oxide."
Further info taken from the second link above
"Sodium cannot be contained in a glass tube. The sodium would etch the glass and further degrade light output. Sodium must be contained in a metal container. Most lamp manufacturers use a special ceramic material known as polycrystalline alumina (PCA) to construct the HPS arc tube. PCA is basically an aluminum oxide material virtually insensitive to sodium attack. PCA tube materials do not lend themselves to the molten sealing method used in the construction of mercury vapor and metal halide arc tubes. Instead, PCA end caps, using either a wire-out end seal or a compound (shrink-fit and cemented) end seal, are epoxied or glued to the tube body using silicone glass. Each tube end cap contains an electrode. The sodium- mercury amalgam and starting gases are placed inside the arc tube before it is sealed closed."
One final component to consider then:
"Unlike mercury vapor and metal halide lamps, HPS lamps are excess amalgam lamps. This means there is more sodium and mercury arc metal placed inside the tube than can be vaporized during start-up and operation. The amount of amalgam that vaporizes depends on the total energy in the arc and the temperature of the amalgam."
"When HPS lamps were first introduced, the amalgam not held in a vaporized state remained condensed in an external reservoir located in the coolest part of the lamp. If the lamp was vibrated by winds or passing traffic, amalgam from the reservoir would splash down onto the arc tube, causing a thermal shock that would extinguish the lamp." ..... "Because of this thermal blink-out problem all but one of the major HPS lamp manufacturers have abandoned the external amalgam reservoir design in favor of internal reservoirs that do not create a thermal blink-out condition."
Now that all the factors involved are covered you could look into spectrum shift or light degredation;
First for spectrum shift as related to a diminishing blue, would relate to how the mercury behaved over time as that's the primary blue source for all HID's.
LINK (wiki)
"Mercury vapor lamps do burn out eventually as the burner electrodes wear, increasing the arc gap. As the lamp nears the end of life, lumen depreciation becomes noticeable and the light given off has a greenish tinge to it. This comes about because the emitter is deposited as a film darkening the arctube wall and reducing light output"
For HPS bulbs (taken from second link in post)
"Material discharged from the electrodes during start-up and operation redeposits on the arc tube ends in much the same way the tungsten filament of an incandescent lamp evaporates and blackens the bulb. This blackening of the arc tube also will increase operating temperaturesand voltage across the arc tube"
"The operating voltage of HPS lamps increases about 1-2 volts per 1000 hours operated. The life of an HPS lamp is dependent on the rate of lamp voltage rise. Lamp voltage will rise until it reaches the limit of the ballast voltage available. At this point, the HPS lamp will cycle ON and OFF, and its effective life will be over."
"Light output from all types of HID lamps gradually declines over time. Lumen maintenance depends on a number of light loss factors. These include any physical changes in the lamp, such as electrode deterioration, blackening of the arc tube or bulb, shifts in the chemical balance of the arc metals, or changes in ballast performance"
The last thing that I would highly recommend everyone to look at is how a manufacturer determines bulb life and "end of life". Since, from an electrical perspective, in all cases voltage rises with age as a result of items pointed out above. A great way, if not the best way, to determine bulb replacement; would be via testing the line voltage for a "fully lit" operational bulb. I'll provide the link again, but is also the second link in the post.
LINK
Refer to page 9 for the voltage range of various lamps and page 10 for lumen drop over time with various lamps.
From the chart; a 1,000 w HPS bulb would need replaced when the socket voltage met or exceeds 350 V.
Hope this helps and sorry to have a long post, but this does condense things down to important aspects I suppose.