I installed a 70 watt HPS light outside back in late September, however I have been noticing lately, 6 months later, that it looks much dimmer than when it was first installed. Do HPS lamps suffer from lumen depreciation at all? I figure that since the lamp is on dusk to dawn about ~10 hours a day, times 6 months, it has about 1,800 hours of life on it. I believe HPS lamps are typically rated at 24,000. Is this a normal occurrence?

It is strange, something should be wrong there.
Does the color shift towards the monochromatic as well?
It may be ballast (lower lamp current), air leak into the outer (bad thermal insulation), generally dirt in the fixture...

I did a visual inspection of the lamp. There is noticeable blackening at the arc tube ends. Seems pretty early in the lamp's life for that to be happening. 

SON lamps do suffer from lumen depreciation, as with any discharge lamp.  Given that there's no phosphor to degrade or anything, I imagine that a good portion of this is due to blackening of the tube ends.  This in turn results in more heat being absorbed, therefore raising the sodium vapour pressure.

In smaller tubes I imagine that the blackening would have a larger impact on the light output of the lamp, simply due to the smaller surface area of the arc tube.  However this definitely shouldn't be a major issue only 2000 hours or so into life if it's not been switched excessively.

Is the lamp of a known brand?  Or are we dealing with a cheap and nasty version?  I've heard some pretty dire things about some of the off-brand imported SON lamps recently, so would tend to stick with those from the likes of Philips or Osram. 

If it is an off-brand lamp, first thing I would try would be fitting one that's from a known manufacturer - but check first that the ballast is of the correct type, and that it's correctly matched to the supply voltage.  For instance a 220V ballast running on 240V will result in the lamp being slightly overdriven.  Likewise in the other direction, this might be especially true if the light's attached to an outbuilding or something from a long supply cable - I'm not well versed enough on SON technology to know how tolerant of over/under driving they are I'm afraid.

SON lamps do suffer from lumen depreciation, as with any discharge lamp.  Given that there's no phosphor to degrade or anything, I imagine that a good portion of this is due to blackening of the tube ends.  This in turn results in more heat being absorbed, therefore raising the sodium vapour pressure.

In smaller tubes I imagine that the blackening would have a larger impact on the light output of the lamp, simply due to the smaller surface area of the arc tube.  However this definitely shouldn't be a major issue only 2000 hours or so into life if it's not been switched excessively.

Is the lamp of a known brand?  Or are we dealing with a cheap and nasty version?  I've heard some pretty dire things about some of the off-brand imported SON lamps recently, so would tend to stick with those from the likes of Philips or Osram. 

If it is an off-brand lamp, first thing I would try would be fitting one that's from a known manufacturer - but check first that the ballast is of the correct type, and that it's correctly matched to the supply voltage.  For instance a 220V ballast running on 240V will result in the lamp being slightly overdriven.  Likewise in the other direction, this might be especially true if the light's attached to an outbuilding or something from a long supply cable - I'm not well versed enough on SON technology to know how tolerant of over/under driving they are I'm afraid.

I'm not sure what the lamp brand is. It came with my Lithonia yard light, so it could very well be a cheapo lamp. The ballast is a simple R-NPF ballast running on 120V.

SON lamps do suffer from lumen depreciation, as with any discharge lamp.  Given that there's no phosphor to degrade or anything, I imagine that a good portion of this is due to blackening of the tube ends.  This in turn results in more heat being absorbed, therefore raising the sodium vapour pressure.

The raising vapor pressure lead to the rise of the arc voltage and this (on most ballasts) to rise of the real power delivered to the lamp.
And the increase of the real power input usually compensate the efficacy depression (due to arctube blackening) over most of the lamp life, so the HPS system does not exhibit as severe (or no at all) lumen dropout.
By the way it is this mechanism, what lead to cycling at the end of life: The extended blackening made the positive feedback (higher temp -> higher pressure -> higher V -> higher P -> higher Temp) so strong, then the system become thermally unstable (the arc voltage rise so much, the ballast can not sustain the arc).

But anyway, i would expect any change being noticeable after at least 1/3 of the lifetime (8+khour), not after only 2k (10% of rated life).

@Under/overdriving (using wrong lamp vs ballast combination; may be even due to error during lamp manufacture):
When overdriving, i would guess the lamp would start to cycle (arctube overheat -> too high arc voltage -> ballast does not sustain the arc)
When underdriving, there is not much reason for severe blackening, as cathodes are not loaded as much

Does the lamp still has vacuum in it's outer (is the getter silvery or white)?
The air inside the outer would cause the thermal bridge carrying too much heat from the arctube. So the arctube would not reach required operating temperature, so the sodium pressure would be low, so the real power delivered to the lamp, so low light output.

The Lithonia HPS fixtures sold at home centers and hardware stores are really made by Caster in China.  The lamp in this case is not particularly high quality.

I have a few of those lamps and have compared them to Big Three (GE/Sylvania/Philips) lamps.  Even with low hours, they are noticeably dimmer, their spectrum is slightly narrower and they tend to blacken quickly.  One that came in a 150 watt HPS wallpack lasted only 2 year or so before it started cycling.

If it still does the job, I'd use it till it croaks.  Than I'd replace it with a Big Three lamp.  Also, be advised that the ignitors in these fixtures can be problematic.

The Lithonia HPS fixtures sold at home centers and hardware stores are really made by Caster in China.  The lamp in this case is not particularly high quality.

I have a few of those lamps and have compared them to Big Three (GE/Sylvania/Philips) lamps.  Even with low hours, they are noticeably dimmer, their spectrum is slightly narrower and they tend to blacken quickly.  One that came in a 150 watt HPS wallpack lasted only 2 year or so before it started cycling.

If it still does the job, I'd use it till it croaks.  Than I'd replace it with a Big Three lamp.  Also, be advised that the ignitors in these fixtures can be problematic.

Good point. I've also considered going the SBMV route when this one fails. Also, of the "Big Three", which brand is considered the best? Or is it subjective?

I don't think it makes much difference.  I think Philips maybe the only one of the three that still makes that size lamps in the US.  GEs are now made at the old Tungsram factory in Hungary.  Sylvania's are made in Mexico, and I've seen some really misaligned arc tubes in them.  Of the three, I'd probably go with the Philips, with GE running a close second.  The Sylvanias are fine if you can find one with a straight arc tube (they sell them in glued boxes).

As far as the fixture goes, I have a similar one branded as Heath Zenith lighting my yard.  Though it's 11 years old, It really only has about 10 months of dusk-dawn use on it.  Though it works fine, I've had two wallpacks that use the same model ignitor go dark due to a failed ignitor.  If that happened, it would be simple enough to bypass the ballast and install an SBMV.

Can you get Osram HPS in the US?, I find the best HPS lamp here are Osram, its what Cheshire highways use in all their street lighting over here.

Can you get Osram HPS in the US?, I find the best HPS lamp here are Osram, its what Cheshire highways use in all their street lighting over here.

Osrams in North America are sold as Sylvania.

My one long term experience with Mexican made Sylvania HPS lamps isn't too great so far.  3 years ago, I relamped an entry fixture at the local fire station with a Sylvania lamp made in Mexico.  This last weekend I noticed it was already cycling.  The lamp it replaced was a 90s US made Philips.

Lamps of US market are made separately, as nearly all wattages differ in arc voltage, so ballast requirement (US version vary in arc voltage, European versions are all in the 70..90V range) anyway.
And if you want to import HPS from Europe, the only chance you have is 150W S56, as only this type happen to be electrically equivalent.

Not necessarily.  Lamps made in Europe can be made to North American specs, and visa versa.  Recently, most smaller General Electric HID lamps are made in Hungary and imported to the US.  Also, before their purchase of Sylvania, Osram did market and sell lamps in North America.  I have even seen a few more recent Osram lamps here made to North American Specs, though those were made in China.

I mean what market were lamps intended for, not where they are really made.
As an individual buyer you would likely not be able to buy any lamp intended for US market in European distributor and vice versa.

My one long term experience with Mexican made Sylvania HPS lamps isn't too great so far.  3 years ago, I relamped an entry fixture at the local fire station with a Sylvania lamp made in Mexico.  This last weekend I noticed it was already cycling.  The lamp it replaced was a 90s US made Philips.

US made doesn't necessarily mean quality. Lamps are rated in time to 50% failure out of a large sample.  There are some that will fail at 20% life which could be that one you got and, some that will last 200%(its your lucky day) , but the survival rate follows a inverted S curve.  You can't judge the quality based on anecdotal evidence from ONE sample life data.