For other uses, see Lambda (disambiguation).
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The Lambda logo (λ) is a symbol found frequently in the Half-Life and Portal universe. It represents the Greek letter "l" (lowercase "L"), and is a radioactive decay constant used in the half-life equation.
In the original Half-Life, Gordon Freeman's trademark HEV Suit was marked with a Lambda Logo on the chest, as were other HEV Suits. A section of Black Mesa dedicated to teleport research was also called the "Lambda Complex", and is identifiable by the Lambda logo at its entrance.
 Half-Life 2 story arc
Gordon's Lambda logo on his suit and the work of the Lambda Complex became something of a legendary trademark symbol for the Resistance against the Combine, referring to the acts of Gordon Freeman during the Black Mesa Incident, and turning the symbol into an informal logo for the Resistance, reminding of the real world Anarchist symbol, of similar purpose. They would spray-paint it near supply caches, and resistance bases and safehouses such as White Forest, Black Mesa East, and Kleiner's Lab. Resistance fighters also wear the symbol on their arm bands. They are also used to identify Resistance-friendly Combine weapons such as Hopper Mines and Sentry Turrets, seen by the spray-paint on them.
The Lambda logo can be seen on certain vehicles, such as the orange van that Dog throws to the Citadel in Episode One although any van-type vehicle, regardless of color, has a Lambda logo under the right tail-light. The lambda is also seen in graffiti and propaganda posters across City 17.
 List of exact in-game locations
The following is a list of all the Lambda caches that can be found by the player in Half-Life 2; they count towards the Lambda Locator Achievement.
Cache 1/45 (
d1_trainstation_05): The first Lambda logo can be spotted near the entrance to Kleiner's Lab, on the right side of the vending machine Alyx leads Gordon to. As it simply marks the Resistance outpost, no items are be found there.
- Cache 2/45 (
d1_canals_01): The first cache of this chapter can be found in the building after the player jumps off the moving train. After breaking the boards off the doorway, the cache can be found to the left.
- Cache 3/45 (
d1_canals_01): Another cache can be found during the escape from the Civil Protection officers and cannot be missed. Keep heading on the regular path and an air duct will be in the way. Break the vent with the Crowbar and the cache will be located inside.
- Cache 4/45 (
d1_canals_01a): At Station 1, Civil Protection units will be guarding most of the base. After they're all dead, the player will have to destroy the supports on a board platform to get across to the other side of the level. An explosive barrel is located on the platform to destroy the supports. Making your way across and dropping down into the tunnel near the rubble, the player will see a Lambda symbol to the left near a fallen metal sheet. Under the sheet is the cache.
- Cache 5/45 (
d1_canals_02): During Gordon's first meeting with the Hunter-Chopper, the player will have to take shelter inside a sewer area. The door to the sewer is blocked by some barrels, but the player can pick these up and remove them. Head down the stairs and the player will see a Lambda symbol under the gantry. Head down the ladder and the cache is located near the symbol.
- Cache 6/45 (
d1_canals_03): When you get past the large metal pipes with steam ejecting from it, the player will see a Lambda symbol to the left and some boards blocking it. Break the boards and head under the pipe. Reaching the end of the pipe will reward you with the cache.
- Cache 7/45 (
d1_canals_05): Sooner or later, the player will reach a curved room with some bare scaffolding on the top. Take the ladder up to the scaffolding. If you continue walking without falling off the scaffolding, the player will be treated to a cache.
- Cache 8/45 (
d1_canals_05): The last cache of this chapter can be found immediately after dropping down to the area with radioactive waste. On the left are some wooden planks which will allow the player to reach the pipe above. At the end of this pipe is the cache.
- Cache 9/45 (
d1_canals_06): Very shortly after the player receives the Airboat and gets past the next loading screen, the player will find another Lambda symbol on the wall. Exit the Airboat and take the ladder up to the gantry and the cache is located atop the gantry
- Cache 10/45 (
d1_canals_06): Once at the red barn, head inside and the player will be treated to some Zombies inside. Kill them and head up the broken stairs where there will be some wooden scaffolding. Head across this and stop as soon as getting towards the two other planks and take the one to the left, as the right one breaks. The player will be greeted to a Headcrab falling from the ladder. Kill it and head up the ladder and the player will find a container full of items on a crane and barrel stuck in-between a winch. Hit the barrel with your Crowbar and soon after, the items will drop into the water for grabs.
- Cache 11/45 (
d1_canals_06): Soon after you beat the barrel puzzle, the player will spot a Combine Dropship heading to the left. The player will spot the Dropship landing to drop off some CP units. Take care of them and the player will see a Lambda symbol on the wall, right next to a broken grate. Use the barrel to get inside the the grate and head into the water. At the end will be the cache, as well as a surprise Zombie.
- Cache 12/45 (
d1_canals_07): After heading out of the CP outpost, the Player come across 3 APCs. When you reach the second APC, the player will see a Lambda symbol on the wall. Get past the rocket firing APC and under it will be a broken grate with a hole. Head into this hole and the cache can be found next to another grate.
- Cache 13/45 (
d1_canals_08): Soon after getting past the third APC, the player will reach a tunnel. At the end of the tunnel, the player will see a basket with some supplies. The basket is connected to another basket with cinder blocks inside. Taking these cinder blocks out will cause the other one to fall, dropping the supplies.
- Cache 14/45 (
d1_canals_08): Soon after the Hunter-Chopper starts to chase you, the player will come across a bridge with a train on it. Stop the Airboat under the bridge and under the bridge will be a cache on the left side.
- Cache 15/45 (
d1_canals_09): During the Hunter-Chopper chase, the player will eventually head through a tunnel with partially submerged pipes in radioactive waste. At the end of the tunnel is a Lambda symbol next to a supported platform. Break the supports with the Airboat and the contents will drop.
- Cache 16/45 (
d1_canals_10): The chase continues and the player will pass through a wide channel with some steep angled sides. Just before a large red building above the player and to your left will be some distinct green graffiti. Use your Airboat speed to get up to the short passage and the cache is located in the small area.
- Cache 17/45 (
d1_canals_10): Upon reaching the bridges with Barnacles, look to the left and the player will spot two dark areas underneath the bridge. The cache is located to the right side.
- Cache 18/45 (
d1_canals_12): After you receive the Airboat gun upgrade, the player will pass under a couple of bridges and come to a fork underneath a tall building with red lettering on top. Take the path to the right and in the tiny area is the cache, along with some Headcrabs and Zombies.
Cache 19/45 (
d1_eli_01): The first and only Lambda symbol in this chapter is where Eli's lab is located. It can be found right next to the ladder up to the lab. As it is associated with a Resistance base, no cache can be found.
- Cache 20/45 (
d1_town_01): Shortly after meeting Father Grigori for the first time, the player will enter a building in order to shut off an electrical fence. Inside are some green generators, one with an air duct located above it. Break the vent and it'll lead outside where the player will find a cache.
- Cache 21/45 (
d1_town_01): Later in the chapter, the player will soon encounter car crushing traps. Using the switches provided, one can get atop the car and ride it up. The player will spot a Lambda symbol below a fence. Hop over the fence and head into the enclosed corner. The cache can be found here.
- Cache 22/45 (
d1_town_01a): Shortly, the player comes across Father Grigori again. This time, he'll be shooting Zombies, opposite from Gordon's position. Clear the area of Zombies, from top to bottom, then climb a ladder to proceed atop the building. At the top, walk to the end of the beam and turn left, where the player will see a hole in a roof with a Lambda inside. Deep inside is the cache, as well as a Fast Headcrab.
- Cache 23/45 (
d1_town_05): The final cache of the chapter is located immediately after the end of the Ravenholm mine shaft. Above the archway are some supply crates, accompanied by a Poison Headcrab. Behind some ducting is the cache.
- Cache 24/45 (
d2_coast_01): Once acquiring the Scout Car, follow the faint track in the sand up a hill and the player will come across a gap between a bridge. Use the turbo speed to get across the bridge. Around the next corner is a Lambda symbol marking this location, clearly visible on a red shack at the side of the road. Inside the shack is the cache.
- Cache 25/45 (
d2_coast_03): At the sight of the first Thumper, exit the Jeep and there will be a small house next to the Thumper. It appears to be locked from the inside, but the house can be accessed via the cellar. A Lambda symbol can be seen obscured by tires. Break the cellar door and continue into the cellar. Inside will be a Grenade crate and a Poison Zombie, which can be taken out with the Grenades provided. Once it has gone quiet, the cache can be reached by stacking crates and barrels to get above.
- Cache 26/45 (
d2_coast_04): After the Gunship battle at NLO and the crane segment at Dock 137, another Gunship will be encountered after jumping over the bridge. Deal with the Gunship using the crate of rockets in one of the cars blocking the road ahead. Soon after the Gunship is down, head on the road and stop at the blue van. A Lambda symbol is marked at this location. The van contains the cache.
- Cache 27/45 (
d2_coast_05): After driving for some time, the player will come across a Petroleum Station with a surplus of Combine Soldiers and a Force Field blocking the Scout Car from entering. The force field can be disabled by removing the chocs under an APC. Near the APC will be a fenced area with a white gas tank and a Lambda symbol. Use a barrel to get inside the fence, which has is partially torn down. Head inside the enclosed area and inside is the cache.
- Cache 28/45 (
d2_coast_07): The last location in this chapter can be found right after the previous map. After exiting the tunnel, a Dropship will be seen carrying Combine Soldiers from a large house in the distance. Upon approaching this house, check the jagged outcrop of rocks. Enclosed behind the rocks is a small area blocked by boards. Head inside and inside is the cache.
- Cache 29/45 (
d2_coast_09): After completing the bridge portion and heading out of the tunnel, continue until some oil tankers start blocking the path. An unopened gate is blocking the Scout Car from entering, but it will open after finding 3 Batteries in the area. Above the doorway leading to the toilet is a Lambda symbol on the roof. Use the Gravity Gun to grab the single Energy Ball canister.
- Cache 30/45 (
d2_coast_11): The second and last cache of this chapter can be found during the sand puzzle segment. In the open area across the rocky outcrops, a wooden shack with no roof can be seen. The easiest way to reach the shack is by using the long wooden plank in the neighboring outcrop. Once reaching the shack, start climbing above the shack by stacking barrels and crates. Once reaching the top, a white shack with a Lambda symbol can be seen, which is accompanied by the cache.
- Cache 31/45 (
d2_prison_02): Not far into Nova Prospekt is the first cache. Once reaching the area with 2 Turrets, the player will spot a Lambda symbol next to a door. Break the planks on the broken glass and the cache will be inside.
- Cache 32/45 (
d2_prison_03): After spotting the dead Vortigaunt on the chair, head upstairs to the next floor. Beyond the tripwire, the player will see a Lambda symbol marked near a door. The cell door next to it can be slid open to reveal a secret cache.
- Cache 33/45 (
d2_prison_05): The final location in this chapter is in the laundry room with the ambush set up. In the room, the player will see a small passageway over the short railing to the right. Hop over the railing and the cache can be located in the small room. Stay sharp, as a Poison Zombie is in one of the rooms.
Cache 34/45 (
d2_prison_06): The only cache in this chapter is found right after Gordon and Alyx split up again. She'll open a blocked gate for you and in the area beyond is a vending machine. Atop the vending machine is a small cache.
- Cache 35/45 (
d3_c17_01): Immediately after emerging from Kleiner's lab with Dog, to the left, the player will see a dumpster with a crate near it. Use the crate to get atop the dumpster and the cache will be seen easily visible next to the wall.
- Cache 36/45 (
d3_c17_04): A little while after encountering the Combine Hoppers, the player will enter a courtyard with Civil Protection units firing at the player. There is a surplus of explosive barrels, with one near a tiny red shack. Behind the barrel is a metal sheet, with the cache located inside.
- Cache 37/45 (
d3_c17_05): After running through a series a buildings, the player will come through a underground tunnel where Turrets stand watch over a hole in the wall. Through the hole, the path will split into 2 individual paths, one leading upstairs and one leading to the next area. Head up the stairs and the player can see a cache behind a blocked gate. Use your Gravity Gun to get the supplies behind the gate.
- Cache 38/45 (
d3_c17_06a): An underpass full of Manhacks will be ready to attack the player. In the second segment of the underpass, after going around a wall of rubble, turn back towards the collapsed roof. The cache can be found near the glowing fire.
- Cache 39/45 (
d3_c17_06b): After one of the Resistance fighters lets down a bridge for Gordon, keep going until encountering another bridge. If the location is missed, the Lambda cache is right behind the wall inside the ruined building.
- Cache 40/45 (
d3_c17_06b): The player will spot this Lambda as descending through an infested building full of Zombies and Combine Soldiers. At the lowest elevation, the player will spot a Lambda through a doorway. The cache is blocked by a fence, which can be accessed using the Gravity Gun.
- Cache 41/45 (
d3_c17_08): After a standoff with Alyx in the plaza area, the player will proceed to an underground area. Head forward until reaching a room with a bottomless pit. A vertical pipe can be found going to the lowest point of the level. the player will see the cache in a small area.
- Cache 42/45 (
d3_c17_08): The player will come across Combine fighting off zombies, this time in a sewer looking area. After dropping down into the lower area, a passageway leads off to the left. Follow this, and shortly the player will come across a ladder to the right. Ignore this ladder and continue onward. The player should notice the Lambda symbol ahead, where the passage splits. The cache is hidden in a hole in the wall, which can be obtained by using the Gravity Gun.
- Cache 43/45 (
d3_c17_08): This cache is very close to the previous cache. Standing in front of the last cache, look to your left and the player should be able to see another passageway with capped metal pipes inside. Climb over the pipes and the cache can be found just ahead.
- Cache 44/45 (
d3_c17_08): The last location in this chapter can be located in the open room with the lift. Fight to the top, but before dropping down to the next level, navigate around the red corrugated building using the narrow ledge. The player should see a faint Lambda symbol behind a wall. The cache will be in the same area.
Cache 45/45 (
d3_c17_12b): The final cache in the game can be found very far into the chapter. After getting to the building with a Soldier ambush and the Strider under the tunnel, head up to the highest level and try to avoid Strider fire if possible. After getting past the area, the player will find a bunch of crates and should unlock Lambda Locator if all previous caches have been obtained.
 Related Achievements
|Lambda Locator (15G)|
|Find all lambda caches in Half-Life 2.|
|Half-Life 2: Episode 2|
|Cache Checker (10G)|
|Find every radar cache in chapter Under The Radar.|
- The Lambda symbol is used in the half-life equation to represent the decay constant (i.e. the rate of radioactive decay in an element).
- The Lambda logo is also used to denote the wavelength of a sound or light emission.
- Lambda is used in some programming languages and parts of math to denote a function.
- The Lambda logo can be found as an Easter egg in both Left 4 Dead games. In Left 4 Dead Francis has it shaved into his hair. In Left 4 Dead 2, a Charger zombie is seen hanging from posts in a pose exactly like the lambda logo.
- The Lambda symbol looks not unlike an arm holding a Crowbar, which is of course Gordon's signature melee weapon.
- The Lambda symbol can be found on Dog's back.
The Lambda Reactor Complex logo.